the odds of him bluffing this board in a 3-way are slim to none, and there's not many draws he can have. if the SB is a fish it's best to call the first bet to keep them in. Stacking off with as much equity as you do vs 99+{-TT} can't be a mistake, but it may be more +EV to just call the flop (regardless of SB's fishiness)