i three bet for two reasons.
1. To define it opp wants to play for stacks preflop or wants to see a flop. Therefore i know how good my hand is and what his range thus is.
2. Isolation or encouragement to push from a weaker but still strong hand.

Eg. In a raised-reraised pot id 3 bet TT-KK for isolation and information.
In a mass limped pot with an lp raise my range is wider for isolation only.