Quote Originally Posted by Mezza Morta
3-bet any hand in the top half of your opponent's opening range.
This usually isn't best. Refer to the discussion of 3-betting in terms of Renton's ABCD subrange concept in the FR forum for more information.

Edit: I decided to come back and give an example to get things started in the right direction a bit more smoothly for you.

1. A 3-bet bluff is not profitable against all Villains in all situations. In these spots, we should be 3-betting for value most, if not all, of the time.

2. Suppose a Villain opens with 99+ and continues against a 3-bet with QQ+, then a 3-bet bluff is not profitable (a quick calculation can confirm), and a 3-bet for value with QQ isn't profitable even though it's in the top half of Villain's open-raising range.

3. In situations where a 3-bet bluff is profitable but you decide to only do so with X% of hands, you should tend to form that X% from hands that aren't strong enough to flat call, but have some sort of post-flop playability for the times that you do see a flop. For example, 74s will perform better than 72o as a 3-bet bluff because it holds more equity in the times you are forced to see a flop.

4. The proportion of times you 3-bet for value to the times you 3-bet as a bluff will give an indication as to if you are vulnerable to 4-bet bluffs.

I hope this makes for a more complete response.