$100NL 6-max @ FT

CO limps, SB completes. i check the BB w/ A6o. CO seems marginally competent. SB is a moron and i'm pretty sure he's colluding w/ the button, but they both suck so whatever. $300 each for me + CO, $600 for SB.

flop: [8h][6h]
pot: $3

check, i bet $3, CO calls, SB CR's to $9.

1. how do we proceed from here?

2. lets say i call, CO calls, and turn is an offsuit 7. SB leads $18 into the $30 pot. how do we proceed from here? SB is an absolute idiot and his range is very wide but CO is kinda ok/good.

3. lets say i 3-bet to $27. CO folds and SB calls. turn is an offsuit 7 and he checks to me.

Fold. Reverse implied odds etc.

3-bet, check behind on the turn, call river. Cheapest way to get to showdown without letting the CO come along.

That would depend on SB range at this point, your read doesn't give anything.
Against certain players I would just fold on flop.
If his range is broader than just 2pairs, call flop, raise his turn lead, check river, fold to any action.