Flush Draw Check, Bet, or Check/ Raise

MP1 is and unknown who just sat down 3 hands ago and posted from the CO position. He has shown done twice with decent hands that he chased and lost with and folded once pf. MP3 is not much better in terms of reads, but is 13/10/2.5 over about 30 hands no showdowns that I remember -- no notes.

PokerStars 1/2 Hold'em (10 handed) FTR converter on zerodivide.cx

Preflop: Hero is UTG with A[img]/images/graemlins/heart.gif[/img], 7[img]/images/graemlins/heart.gif[/img].
Hero calls, <font color="#666666">2 folds</font>, MP1 calls, <font color="#666666">1 fold</font>, <font color="#CC3333">MP3 raises</font>, <font color="#666666">4 folds</font>, Hero calls, MP1 calls.

Flop: (7.50 SB) K[img]/images/graemlins/heart.gif[/img], 9[img]/images/graemlins/heart.gif[/img], 6[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
Hero ?

I want to bet here, but should I? If I bet then MP3 could raise or I could lose MP1 all of which (I think) are -EV I have the one overcard though. If I check then MP3 will probably continue and I can decide to check/raise (or not)but what if he doesn't. Is possible I could get two folds here?

I want to bet here. Should I?