When do you throw away trip Aces?

[Thriller]

PokerStars 0.05/0.10 Hold'em (10 handed) converter

Preflop: Hero is UTG+1 with A[img]/images/graemlins/club.gif[/img], 9[img]/images/graemlins/club.gif[/img]. CO posts a blind of $0.07.
UTG folds, Hero calls, UTG+2 folds, MP1 calls, MP2 calls, MP3 calls, CO (poster) checks, Button folds, SB completes, BB checks.

Flop: (7.40 SB) A[img]/images/graemlins/spade.gif[/img], 7[img]/images/graemlins/diamond.gif[/img], A[img]/images/graemlins/diamond.gif[/img] <font color="blue">(7 players)</font>
SB checks, <font color="CC3333">BB bets</font>, <font color="CC3333">Hero raises</font>, MP1 calls, MP2 folds, MP3 calls, CO folds, SB folds, BB calls.

Turn: (7.70 BB) 2[img]/images/graemlins/diamond.gif[/img] <font color="blue">(4 players)</font>
BB checks, <font color="CC3333">Hero bets</font>, MP1 calls, <font color="CC3333">MP3 raises</font>, BB calls, Hero calls, MP1 calls.

River: (15.70 BB) K[img]/images/graemlins/club.gif[/img] <font color="blue">(4 players)</font>
BB checks, <font color="CC3333">Hero bets</font>, MP1 calls, <font color="CC3333">MP3 raises</font>, BB folds, Hero calls, MP1 calls.

Final Pot: 21.70 BB
<font color="green">Main Pot: 21.70 BB, between MP3, Hero and MP1.</font>

[btspider]

after I see MP3's flush and get to muck my cards. Why did you bet the river?

[Thriller]

I had seen MP3 call everything(almost) from the time I sat down.

[sfer]

[ QUOTE ]
I had seen MP3 call everything(almost) from the time I sat down.

[/ QUOTE ]

Is that a good reason to bet the river after he raised the flush completing card on the turn?

[SoCalPat]

Check/call the river. With three other players in the hand calling the turn raise from MP3, if MP3 doesn't have the flush, it's not improbable that someone else does.

[cartoonsoldier]

3-bet the turn, if he caps, check-call the river.