I can't be ahead

[SGS]

I have 77 in the CO of a 15/30 game at party. The Hijack opens for a raise, I 3-bet, the BB comes along and the hijack caps it, all call. No info on either player.

FLOP 3,3,6 two hearts

Check, bet, I raise, fold, 3-bet, I call.

TURN 4o

He bets I call.

River 6

He bets I fold. I just really don't see any hand I am ahead of here that he pushes this hard. Maybe AKhh, but once he bets the river the chances of that are reduced. Thoughts?

SGS