a5s in BB?

[VickreyAuction]

MP limper is 40/15/2.26. Button is a TAG, 24/18/3.5.

PokerStars 2/4 Hold'em (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

Preflop: Hero is BB with A[img]/images/graemlins/club.gif[/img], 5[img]/images/graemlins/club.gif[/img].
<font color="#666666">1 fold</font>, MP calls, <font color="#666666">1 fold</font>, <font color="#CC3333">Button raises</font>, <font color="#666666">1 fold</font>, Hero calls, MP calls.

Flop: (6.50 SB) 5[img]/images/graemlins/diamond.gif[/img], 4[img]/images/graemlins/diamond.gif[/img], 9[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
Hero checks, MP checks, <font color="#CC3333">Button bets</font>, <font color="#CC3333">Hero raises</font>, MP folds, <font color="#CC3333">Button 3-bets</font>, Hero calls.

Turn: (6.25 BB) 3[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">Button bets</font>, Hero calls.

River: (8.25 BB) 9[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">Button bets</font>, Hero calls.

Final Pot: 10.25 BB

Preflop, flop? If it were heads up on the flop, I would at least consider a passive line, but 3way I wanted to knock out MP with what was likely the best hand. I have to call the turn with my wheel draw, but do I fold the river?