Flambae is probably right hand dominant (throws a punch with it 3x, opens doors with it, beckons Robert to hurry up in the meeting room with it, slaps a takeout box with it, pats Robert's head with it, holds a microphone with it, eats with it, shakes hands with it, adjusts/touches his comms earpiece with it). he seems to primarily use his left hand when he's alternating hands for a task naturally, or he's feeling self-conscious, hostile, or defensive (taking off his glasses after opening a door, opening the fridge to reach inside, squeezing Robert's shoulder after patting his head—then when Robert asks, "Remind me, who took your tooth/burned off your eyebrows?" Flambae points with his left hand when he says, "i got in a fight with an actual real actual hero, and i assure you he's regretting his decision to tangle with me", and he points at the team with his left hand after the Mecha Man reveal as he says "Fuck all you fucking fucks")
which means if you're writing him having sex and he's fingering his partner, there is probably NOT a third finger!! ✌️ just these two. and a thumb. we can get creative with it, folks!