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How often does SB 3-bet here? We have no idea SB is going to 3-bet, and we have POOR relative position because MP2 is the aggressor, so if we raise the turn, we knock out the rest of the field. If we know SB will 3-bet then of course we call then cap, but why will SB 3-bet? We don't even know that BB would 3-bet.

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There are two separate situations here.
1. The original choice to 3 bet.
2. The choice to cap, when SB check\3bets.

In choice 1.

I think SB, BB and MP2 are way more likely to fold than raise, or call, for that matter. So what hands that we can, are we trying to knock them off of? For every person we knock out, our edge reduces. AND we stand to leave ourselves with crappy position to the raiser when we make our hand.

THIS IS AN IMPORTANT CONCEPT <font color="blue">
Because this hand does not improve by knocking people out:</font>

ie. 54% equity stays the same.
4 villians 34%edge
3 villians 30%edge
2 villians 21%edge
HU 4%edge

This only stands to magnify losses when you knock people out with a hand like this. Because <font color="red"> you are losing money on bets that are ALREADY IN THE POT </font>as well as revenue lost from those that you knocked out.
<font color="purple">
You only want to knock people out, when doing so INCREASES your equity.
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Choice 2. We DO have perfect position on the aggressor.