Sucker's straight being raised

MP2 is a complete fish, he hasn't raised one pot the hour I've been at this table, until this one. So 3-betting the flop is obvious, but seeing that fish hadn't raised yet today and the turn pairing the board both scared me. Should I continue betting or was my play justified by the scare card and the fact that fish hasn't raised a pot yet?

Thanks for the input

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

Preflop: Hero is BB with 8[img]/images/graemlins/diamond.gif[/img], 4[img]/images/graemlins/heart.gif[/img].
<font color="#666666">4 folds</font>, MP2 calls, MP3 calls, CO calls, <font color="#666666">2 folds</font>, Hero checks.

Flop: (4.50 SB) 5[img]/images/graemlins/diamond.gif[/img], 6[img]/images/graemlins/diamond.gif[/img], 7[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(4 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">MP2 raises</font>, MP3 folds, CO folds, <font color="#CC3333">Hero 3-bets</font>, <font color="#CC3333">MP2 caps</font>, Hero calls.

Turn: (6.25 BB) 7[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">MP2 bets</font>, Hero calls.

River: (8.25 BB) 3[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">MP2 bets</font>, Hero calls.

Final Pot: 10.25 BB

[LImitPlayer]

I lead out on the turn here, even after the flop was capped.

On the flop there is only 1 hand that beats you. 89

If the turn is raised i just call down and check call the river.