This may have been the toughest river decision I've ever faced. See what you do:

SB is 31/10/1.4 after more than 1000 hands
Button is 75/5/0.8 after ~100 hands

Party Poker 10/20 Hold'em (6 max, 5 handed) pokerhand.org hand converter

Preflop: Hero is BB with 3[img]/images/graemlins/heart.gif[/img], 5[img]/images/graemlins/spade.gif[/img].
<font color="#666666">2 folds</font>, Button calls, SB completes, Hero checks.

Flop: (3 SB) 5[img]/images/graemlins/diamond.gif[/img], 7[img]/images/graemlins/diamond.gif[/img], 5[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">SB bets</font>, Hero calls, Button calls.

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

River: (3 BB) K[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, <font color="#CC3333">Button raises</font>, <font color="#CC3333">SB 3-bets</font>, Hero????