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EDIT: Looking again, you should have capped the flop when it came 3 back to you.
I still am strongly opposed to 3 betting. 3 betting this flop is just plain crazy talk.

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This makes no sense. You've got a cold-caller between yourself and the first raiser, so at least those two players are going to stay in. With JUST the flush draw you have enough equity to raise for value against 2 opponents (I discount the possibility of two-card flush over two-card flush almost entirely, as rare as it is; if the flush comes in and somebody else is still going nutbag, we worry about that, then). Add in the nut OESD and you've got enough equity to raise for value with just 1 opponent (you're better than 1:1 to make your hand by the river).

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The fact that you've discounted the flush over flush only further strengthens the argument not to 3 bet.
I think you are strongly overestimated the chance that SB,BB,MP2 are all coming along for 1,2,2 bets more respectively (especially now you've discounted the flush).
<font color="blue">
If Hero 3bets </font>and SB folds, BB folds and MP2 fold(because their both facing a possible cap from MP3) it looks like this.
2sb dead money + 3sb from MP3 and CO = <font color="blue"> 8sb = 4sb equity</font>
<font color="green"> If Hero calls.</font>
sb folds, BBcalls, MP2 calls. = <font color="green"> 8sb = 4sb equity</font>
<font color="purple">If Hero 3bets </font> and a realistic best case scenario- all call.
= 12sb = 6sb equity.

Hero has 54% equity. <font color="red"> Best case scenario, you make an extra 1BB on the flop. </font>


Hero is 54% to make his hand by the river and 32% to make it on the turn. This increased likelyhood of making his hand on the turn is a major factor in wanting to smooth call this flop <font color="red"> AFTER SB 3bets </font> , BECAUSE YOU HAVE RAISELISCIOUS POSITION.
In all likelyhood there is a very slim chance you would normally ever got the chance to cap this flop.


Even if you don't make your hand on the turn. You still want 4 players in. Hero is still 32% to make his hand on the river. If he keeps all 4+ players in he is making another +EV call on the turn and he gets to draw cheaply.
As matter of fact +12%EV*4BB = .5BB
If you had only 2 players left at this point his call is roughly neutral EV and this further reduces the profitability of the flop 3bet.

There are many times when a 3 bet on the flop with the straight flush draw is the right move.
IMO This is not one of them.

There have been a few cases of this overly aggressive 'edge pushing' on the flop recently.
<font color="green"> Kwazzie theory is</font>: Never take the initiative away from an aggressor on the sb street, when you have perfect relative position on the BB streets.