$100NL weird spot 3-way 300bb deep

[ua1176]

$100NL 6-max @ FT.

CO limps. he's marginally decent. lets say 22/13/2 (i dont have PT running at this point in time though). SB completes. he's mega-donk. like 50/25/2. i check in the BB w/ A6o. 300bb each for me + CO, 600bb for SB. pretty sure SB is colluding w/ Button...but they both suck so whatever.

flop Ac 8h 6h
pot: $3

check, i bet $3, CO calls, SB CR's to $9. how do we proceed from here?

if we 3-bet flop, do we call a 4-bet?

if we just call his CR, and CO calls too, and the turn is an offsuit 7, how do we proceed when SB leads 2/3 pot?

all thoughts appreciated. thanks.

jon