对于一个cap，主要要了解maturity和互换的双方：这里maturity应该是三年；互换的双方可能是fixed rate 5%和float rate 3-month libor + 1.5%，3个月换一次。

No, I don't think so. This is a cap, not a swap.

The premium 1.5% is just the option price - remember the cap is really a summation of a number of caplets, which are like call options on LIBOR. Here 5% is strike. Not sure what "flat" is referring to, though.

其实我想说的是每三个月的payoff是max((3-month libor + 1.5%)-5%)×N
昨天写的时候把它写成swap了