On flush draws one-carded and third-nut

I play it exactly the same.
A value bet on the end HU I can see (both if I have no flush [better hands will fold] and if I have Q+ [worse hands will call]) but into 2 villians I don't.
Betting into 2 villians, I'm guessing only T,J,K,A flushes are playing. You win 1BB from T and J but you're probably losing 2BB to both K and A.

[TomBrooks]

This is six max, I'd be more aggressive and less likely to give credit for a flush than full ring.

TURN: Bet. Semi-bluff with 15 outs.

RIVER: Bet. In six max I think 3rd nut flush is pretty good.