Am I missing value here?

[S. L.]

MP3 is 78/38/1.87 (74 hands)
CO is 50/0/0.75 (10 hands) basically unknown
BTN is 77/40/2.13 (22 hands)
SB is 44/2.7/0.75 (44 hands)

Table later grew to full ring, so stats are somewhat mixed.
No special reads so far, I just had a feeling that MP3 had some maniacal tendencies.


Limit Hold'em, $0.50/$1 (5 handed)

Preflop: Hero is BB with 8[img]/images/graemlins/heart.gif[/img], 7[img]/images/graemlins/heart.gif[/img]
CO posted
<font color="#CC3333">MP3 raises</font>, CO calls, Button calls, SB calls, Hero calls,

Flop: 3[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/heart.gif[/img], 3[img]/images/graemlins/club.gif[/img] ( 4.5BB )
SB checks, Hero checks, MP3 checks, CO checks, <font color="#CC3333">Button bets</font>, <font color="#666666">1 folds</font>, Hero calls, <font color="#CC3333">MP3 raises</font>, CO calls, <font color="#666666">1 folds</font>, Hero calls,

Turn: Q[img]/images/graemlins/diamond.gif[/img] ( 8BB )
Hero checks, <font color="#CC3333">MP3 bets</font>, CO calls, Hero calls,

River: 4[img]/images/graemlins/heart.gif[/img] ( 11BB )
Hero checks, <font color="#CC3333">MP3 bets</font>, CO calls, Hero calls.

My play was pretty passive, but i can't really see where and why should I press it.

Flop: I could lead out as a semi-bluff, but i was figuring MP3 will just raise me and scare people off (which i probably don't want since the only possible threat to my draw are pocket-pairs, right?);
I could go for a c/r if MP3 bet and there were enough callers, but i don't like to do it with non-nut-draws and espetially when the board is paired. So I figured, check/call is fine.

Turn: C/r on the flop was no good news, but I don't really care, at that point I decided I'm going to SD if i hit a heart.

River: that's the main question; would bet/call, check/raise/(fold?) be better?