opening can be valuable as a preempt if LHO has a good hand that's hard to describe in one bid, especially if your partner is able to raise spades.Third seat is the classic time to open with a weaker than normal hand. But when is it safe to open a weak hand? Why should you? How do you escape without getting too high when you do? And when you have a real opening bid -- how do you stop your partner from dumping you cheaply because he's afraid you opened light?
It's an ideal time for very aggressive preempting, because if you have a very weak hand, and your partner was unable to open, you can be sure your opponents have most the strength and are about to start bidding strongly as soon as 4th hand comes into the bidding.
The same argument applies, with reduced force, when you have a 10-12 point hand. Your side might still have more than half deck, if partner had a maximum pass. But if anyone at the table has a good hand, it can only be the person on your left who hasn't had a chance to speak. A 1 opening can be valuable as a preempt if LHO has a good hand that's hard to describe in one bid, especially if your partner is able to raise spades.
You still have to exercise restraint in opening weak hands at the 1-level. Remember, your bidding system still has to cater for all the full opening bids you might hold. You don't want to cripple your own ability to find good games.
For the reasons mentioned above, preempts and weak twos in third seat will tend to run almost a full trick weaker than the same bid in second seat. You're trying to take away as much bidding space as you can, and you have no fear of your partner going crazy and looking for slam with 16 points, or of him coming in with a 7-card suit of his own and arguing with you about what trump should be.
To open at the one level in third seat, you need to have a good reason. What is a good reason?
Do you have full opening bid values? If so, you still have an opening, and should go ahead and make your normal bid. It's only when you have a subminimum hand that you need to worry about whether or not you can afford to bend your system and open.
Will your hand be difficult to describe later with an overcall, if you don't open? If so, you want to describe it now while you have the chance. This is an argument in favour of opening Ax 
AJxxx 
Jxx 
xx. This is your only chance to compete in hearts; if you pass and hear 1-Pass-2 back to you, you're out of luck, even if partner has the cards to let you make 3. On the other hand, there is not so much of a hurry to open xx x KQTxx KQTxx: you are very likely to be able to come back in with an Unusual 2NT call at your next turn.
Is it safe? You still have a partner. If you open a subminimum hand, you normally have to be prepared to pass any response your partner makes. If you take a second bid your partner will assume you have a full opening bid. It is safe to open with AQxxx xxx Qxx Qx: if partner responds with 2 of a new suit, he is denying spade support and showing a decent 5-card suit of his own, and will be happy with your support for him. Of course you hope partner bids 1NT or 2, both of which you will also pass. It is dangerous to open a hand like AJxxx x xx KJxxx. You are not prepared to hear your partner respond 2 or 2. Even if he bids 1NT you'll give him the impression you have more in high cards when you rebid 2.
Are you making the opponents' life difficult? Opening 1 doesn't take away any bidding space at all from your opponents; opening 1 takes away a full level of bidding from them. Opening ATxxx xxx Kxx Kx puts some pressure on fourth seat. Opening xxx Kxx Kx ATxxx doesn't.
Are you prepared to have your suit led? Opening a 4-card major instead of a short club (or even instead of a 5-card minor!) is tempting in third seat, because of the higher-ranking bid's preemptive value. And opening 1 is a better bid on KQJx Kx xxx Jxxx than 1 is, for that very reason. But if you hold KTxx xx AKxx xxx, and your LHO winds up declaring 4, you will be sorry if your partner leads a spade into declarer's ace-queen because you asked him to with a foolish 1 opening. Either pass, or or open in diamonds, the suit you want led..
Responder has to be alert to the possibility of a light third seat opening in selecting his response. In particular, in Standard American, a new suit bid by a passed hand is constructive, but not forcing though opener usually will rebid anyway. This leads to a couple important considerations for responder after a third seat opening:
A two-over-one response requires a good 5-card suit. If partner opens 1 in third seat, you can bid 2 with xx Axx KQTxx xxx, but had better stick with 1NT holding xx AJx QTxxx Kxx. If opener has 11 HCP and a flat hand, you will be playing 2!
Support for a third-seat opening of 1 or 1 must be shown immediately. Opposite a first-seat opening, you can show a 3-card limit raise by bidding 2 of a new suit (in Standard American) or 1NT Forcing (in 2/1), then going back to opener's major at your second turn. If you are a passed hand, you are not guaranteed a second chance. If partner opens 1 and you hold QJx xx AQxxx Jxx, you risk going down in 1NT or 2 with nine easy tricks available had you been in spades, unless your first response shows support. A new-suit response denies holding support for opener!
There are several solutions available:
Prevention: This problem arises far less frequently if you follow the advice in Part I of this article, and open in first seat when you are afraid of an impossible decision on the second round if you don't.
Proper hand evaluation: If you hold Axx Qxx Qxx Qxxx, you have a terrible hand with no ruffing values, and at least one of your side queens is sure to be wasted opposite opener's singleton or doubleton. A simple raise from 1 to 2 or 1 to 2 is more than enough, despite holding 10 HCP and an ace. Add another side-suit jack to this hand, and I'd still only offer a single raise even with 11 HCP because the hand has no shape and so many fast losers.
Fit-jumps: In the previous section, I talked about how a 2-over-1 response by a passed hand had to show a good 5-card suit and something like 9-11 points. Is there such a thing as a hand too good for a nonforcing response to partner's opening, that doesn't have trump support and wasn't worth an opening bid? No! Similarly, are there flat 11HCP hands that are worth jumping to 2NT and getting left there opposite a garbage opener? It's safe to respond a nonforcing 1NT on 10 or a bad 11 HCP if you're a passed hand with no good suit and no support.
What else can a jump to 3 over 1 possibly mean, except a hand with a nice diamond suit and spade support? The ideal hand for this jump is something like Qxx Qxx AQTxxx x or Axx xx KQJxx xxx. With 4-card support for partner's major, I can choose between a fit-jump with a strong side suit or a normal limit raise without.
Similarly, you might choose to use the jump to 2NT to promise a hand with a good 11 HCP and three (but not four) card support for opener and scattered side values, offering him a choice between 2NT, 3NT, 3 of his major, and 4 of his major. In my own experience, however, this hand type is virtually non-existent, especially given my first-seat opening bid style.
Drury: Some people avoid the "3-card limit raise problem" by agreeing to use a purely artificial 2 response to show any hand with 3 trumps that is too good for a single raise. This avoids any danger of getting to the 3-level with insufficient trumps, and it gives added safety to the people who frequently open 4-card major suits in third seat.
One down side of the Drury convention, as discussed in Part I, is that if 2 is artificial, responder now has a difficult problem when he holds Jx Axx xxx KQTxx, ideal for a natural passed-hand 2-over-1. A second minus, shared with all artificial bids, is that the opponents can use your 2 bid to their own advantage and uncover an otherwise unbiddable club fit, as in this deal from Bobby Wolff's bridge column.
There are many different forms of Drury in use today. I personally don't recommend Drury; I prefer to make frequent use of the other 3 methods described in this article, and to only open a 4-card major when I have a very good reason for doing so. But it would be only fair for me to note that a majority of modern experts do use some form of Drury. If you wish to learn Drury, I suggest Karen Walker's Drury page as a good starting point for the simplest and most popular form of Drury.
Knowing whether or not your partnership plays Drury obviously influences how aggressive you can afford to be with opening bad hands in third seat, especially hands with 4-card majors. Also refer back to Part I for a discussion of how whether or not you play Drury impacts your first seat openings, as you think about how easy it will be to find a suitable response if you pass a marginal hand in first seat.
Open if you have a normal opening bid, OR:
Mike Lawrence has written an entire book on passed-hand bidding. It's aimed mostly at 2/1 players so has quite a different flavor in places than my presentation does. It covers Drury variations, 4-card majors, whether or not 1NT should still be forcing, etc., in great detail. Surprisingly it doesn't say much about the idea of deciding in first seat whether or not to open based on whether you anticipate trouble responding to a third-seat opener.
Drury Doubles by Jeff Goldsmith, describes a novel way to handle the second round of bidding if you like to open insanely light in third hand. Not for the faint of heart, but strikingly original.