Weird river decision with 43s

[legend42]

Party 20/40

5 limpers (which is rare), I complete from the SB with 4 [img]/images/graemlins/heart.gif[/img] 3 [img]/images/graemlins/heart.gif[/img].

Flop is A [img]/images/graemlins/heart.gif[/img] 9 [img]/images/graemlins/heart.gif[/img] 6 [img]/images/graemlins/club.gif[/img]. It gets checked around (the big blind open folded). Turn is 9 [img]/images/graemlins/spade.gif[/img]. I check, 2 EP players check, MP1 (164 hands, 29 9 1.1) bets, MP2 calls, button (a terrible player) calls, I call, both EPs fold.

River is 3 [img]/images/graemlins/diamond.gif[/img]. I check, MP1 now checks, MP2 (133 hands, 43 18 2.6 (2.1 river AF)) bets, button folds, I...