There are two reasons to three-bet:

1) for value, i.e. you have a good hand (AA, KK etc.) and would like to get value from worse hands, or

2) as a bluff/semi-bluff - in which case it's known as a light three-bet.

When you three-bet light, you're making a semi-bluff at the pot. You know that your opponent is raising light, you can three-bet him light and have him fold, winning you the pot immediately.

This leads to you winning more pots without showdown as well as getting action on your real, three-bet-for-value-type hands.

But although the practice of three-betting light is commonplace these days, many players still routinely size their three-bets incorrectly.

Some players size their reraises on the strength of their own hand.

They bet a bigger amount when they have a weak hand and want their opponent to fold and bet less when they are betting for value.

This is incorrect thinking. A skilled opponent will pick up on this and exploit you.

Your bet sizing should not be determined by the strength of your hand.

Position Dictates Everything

So if hand strength isn't the deciding factor, what is?

The answer is position. You hear it over and over again - position dictates everything in poker.

For determining the size of your three-bet it's no different. When you're in position you can get away with a smaller three-bet size.

This is because you will be last to act for the entirety of the hand. Since acting last is such a huge advantage, you can punish the out-of-position player often, regardless of your hand strength.

When you are in position a good reraise size would be around 3x to 3.5x the original raise.

It's big enough that your opponent does not have an automatic call, yet it doesn't risk an unnecessary amount of chips.

Upping Your Bets Out of Position

When you're out of position, life is always tough. Your decisions need to be made without the advantage of knowing your opponent's action.

Since he always has the last say, he's in control and you're at a disadvantage.

To make up for this you always want to reraise more from out of position.

Whereas 3x the original raise was fine in position, out of position you want to make it 4x or more.

You essentially would like to charge him for the privilege of playing in position against you.

When you're in position, you don't mind seeing a flop and letting your edge manifest itself. When you're out of position, you want to discourage him from calling as you will often be left guessing post-flop.

Giving your opponent good odds and position is a mistake, so let them know you mean business with larger out-of-position raises. The larger raise helps negate your positional disadvantage.

Remember

In today's games you're going to be three-betting fairly frequently.

If you routinely make mistakes with your three-bet bet sizing, you're ultimately making it more difficult to win.

Keep your three-bets sized properly to your position and to the number of players left in the hand, and you will make it easier on yourself in the long run.