Misunderstanding pot equity

[Tapin]

I won this hand, after misplaying it pretty horribly.

MP1 was a reasonably solid player (35 hands: 21/6/2.00); MP2 was terrible (35 hands: 82/13/0.64).

Party Poker 1/2 Hold'em (9 max, 9 handed) converter

Preflop: Hero is BB with A[img]/images/graemlins/diamond.gif[/img], A[img]/images/graemlins/heart.gif[/img]. UTG posts a blind of $1.
UTG (poster) checks, <font color="#666666">1 fold</font>, <font color="#CC3333">MP1 raises</font>, MP2 calls, <font color="#666666">3 folds</font>, SB calls, <font color="#CC3333">Hero 3-bets</font>, UTG folds, MP1 calls, MP2 calls, SB calls.

Flop: (13 SB) J[img]/images/graemlins/club.gif[/img], Q[img]/images/graemlins/spade.gif[/img], 8[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(4 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, <font color="#CC3333">MP1 raises</font>, MP2 calls, SB folds, <font color="#CC3333">Hero 3-bets</font>, <font color="#CC3333">MP1 caps</font>, MP2 calls, Hero calls.

Turn: (12.50 BB) A[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">MP1 raises</font>, MP2 calls, <font color="#CC3333">Hero 3-bets</font>, MP1 calls, MP2 calls.

River: (21.50 BB) T[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, MP1 calls, MP2 calls.

Final Pot: 24.50 BB

The reason I'm posting this hand is to be ridiculed for my terrible grasp of pot equity. After MP1's raise/call preflop, I assumed he had a very good pocket pair and was willing to give me credit (I'd had a pretty tight table image to that point), so I put him on JJ or TT. MP2 played a2c. So when MP1 raised me on the flop, I assume he'd tripped his jacks (hey, I was pretty close with my read...) but figured that since MP2 was likely to call down with anything (71% WTSD, 35% W$SD) I was right to 3-bet it and hope to hit my 3.5-outter.

After running the hand through PokerStove, though, I was only 10% to win the hand against the JJ that I expected + a2c -- and a2c was 7.5% to win the hand!

Because I obviously need remedial assistance on the topic, can someone tell me if I'm right in thinking that I needed to be at least a 34% chance to win the hand to 3-bet the flop?