Well, now... That's an interesting question. Depends if you mean flammable at the surface or at depth. Lighting them afire at the surface depends on oxygen in the ambient air. At depth, you would need a sufficient amount of ambient oxygen to support combustion - there's some O2 in trimix, but I don't know offhand how well it supports combustion.
FWIW, not all human gas contains methane. There is also some hydrogen produced. Methane burns blue. It's the hydrogen (and a few other compounds) that give the additional orange or yellow hue to the flame of burning bowel gas. Some of bowel gas is from swallowed air, too.
So, my answer/guess is: the gas itself would still be just as flammable. The question would be what would happen if you lit a fart in a pressurized container filled with trimix.