[MRBAA]

When he three bets (or raises) on fifth, he has told you he has a flush or higher trips. It's impossible to have a full house in stud before the river without a pair showing. So if you did not have a pair on the board, he could know that his flush was the boss hand. Now, you have plenty of pot odds to call bets on fifth and sixth (and even the river in hopes of amisplayed aces up) without filling up. But once a sane opponent has shown a willingness to keep raising, you have to slow down -- you an underdog to fill, so getting more money in at even odds is -EV for you.