How Bad?

I think you know the answer already.

Full Tilt Pot-Limit Omaha, $0.50 BB (6 handed) - Full-Tilt Converter Tool from FlopTurnRiver.com
Button ($50.45)
SB ($20.20)
BB ($62.35)
UTG ($97.75)
MP ($77)
Hero (CO) ($52.65)
Preflop: Hero is CO with 6, 5, 3, 4
1 fold, MP bets $1.75, Hero calls $1.75, 1 fold, SB calls $1.50, 1 fold

Flop: ($5.75) 4, J, 8 (3 players)
SB checks, MP checks, Hero checks

Turn: ($5.75) 5 (3 players)
SB bets $5.75, MP calls $5.75, Hero calls $5.75

River: ($23) 7 (3 players)
SB bets $12.70 (All-In), MP calls $12.70, Hero calls $12.70 <--- How bad?

Answer: Pretty bad. If the MP folds, then I'm not really all that upset about calling, since SB could be spazzing with a set or, more likely, jamming his 67 (which I just caught up with). When MP calls, he almost certainly has his own made straight. So now, if I call, I'm either hoping for either a 3-way chop or for MP and myself to chop up the SB's chips.

Total pot: $61.10 Rake: $3
Results:
SB had J, 4, J, A (three of a kind, Jacks).
MP had 9, 10, 10, 9 (straight, Jack high).
Hero had 6, 5, 3, 4 (straight, eight high).
Outcome: MP won $58.10

MP plays it wisely by just calling with the nuts, allowing it to creep into my head that he may just be keeping the aggro SB in line with a low-end straight or even a set. This entices me to make a spewy call behind.

Of course, it's easy to focus on my river play here as the source of my woes, when I should be looking at the bad turn call. I'm drawing slim against a made straight, not much better against a set, and on top of it all, I don't have any flush draw. I'm basically praying for one of 3 non-diamond deuces, and then for everyone else to shut it down. It's hopelessly optimistic and a losing proposition from the start.