Aces up facing raise on river

[pokernicus]

My read is that villain is semi-loose. He tends to raise with a real hand, and just calls otherwise. What are the thoughts on how to play the river -- especially facing the raise. Do I three-bet, call, or fold?

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

Preflop: Hero is MP2 with A[img]/images/graemlins/spade.gif[/img], 5[img]/images/graemlins/spade.gif[/img].
UTG calls, <font color="#666666">3 folds</font>, Hero calls, MP3 calls, CO calls, Button calls, <font color="#666666">1 fold</font>, BB checks.

Flop: (6.50 SB) 5[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/diamond.gif[/img], 8[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(6 players)</font>
BB checks, UTG checks, <font color="#CC3333">Hero bets</font>, MP3 calls, CO calls, Button folds, BB folds, UTG folds.

Turn: (4.75 BB) 2[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, MP3 calls, CO calls.

River: (7.75 BB) K[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">MP3 raises</font>, CO folds, Hero??

Final Pot: 11.75 BB