$55: Turned a set but facing a 3/4pot bet and 3 opponents

[AA suited]

Party Poker No-Limit Hold'em Tourney, Big Blind is t15 (10 handed) FTR converter on zerodivide.cx

MP3 (t990)
CO (t915)
HERO (t975)
SB (t985)
BB (t985)
UTG (t1170)
UTG+1 (t1000)
UTG+2 (t1000)
MP1 (t980)
MP2 (t1000)

Preflop: HERO is Button with 3[img]/images/graemlins/club.gif[/img], 3[img]/images/graemlins/diamond.gif[/img].
MP3 calls t15, CO raises to t45</font>, HERO calls t45, BB calls t30, MP3 calls t30.

Flop: (t187.50) 7[img]/images/graemlins/heart.gif[/img], 9[img]/images/graemlins/diamond.gif[/img], T[img]/images/graemlins/spade.gif[/img] (4 players)</font>
BB checks, MP3 checks, CO checks, HERO checks.

Turn: (t187.50) 3[img]/images/graemlins/spade.gif[/img] [color=#0000FF](4 players)</font>
BB checks, MP3 checks, CO bets t150</font>, HERO ??? [img]/images/graemlins/confused.gif[/img]

No reads.

what's the standard line in this situation, and why?