Why wouldn't it be possible in HS? Any 3D software should work.
But you rarely have game with MMF threesomes, it's generally MFF when there is one. And the ones with MMF are fem protag games when she eventually get gangbang'd, and they are mostly done with DAZ3D.
I played a called No More Secret where you're Male MC and can choose to have threesome with a girl and guy.
Same in Our Red String you can have MMF with you Best friend and old friend. Even if you're dating her.
and a bunch more
It's not that rare for some Male MC let his friend fuck the same girl or your friend let you fuck his girl.
Hell, I can see a option MMF in Being a Dik in the future between Elena & JB. He actually likes that MC finds his GF attritive, and even Elena in ep.9 says they like it when other people find them attractive. She even let him fuck her best friend so it make sense she wanna try the MC, she would of heard stories from the other HOTs you fucked & and wanna try for herself lol.
I see it being optional because we would have to the choice to let the boyfriend join or watch us fuck his gf.