The term bet can be used in a few different ways, but generally means "agreed" or "okay". For example, if someone says "Wanna grab lunch later?" and the other person responds "Bet", it means they agree to the plan. Bet" can also be used as a response to confirm that a statement is true.
The smallest 3-bet you should make is twice the amount of the open raise, but re-raising by a factor of three is a good idea if you're betting in position. If you're out of position, your 3-bet sizing should be closer to four times the open raise. Tighten your 3-bet range.