cracks knuckles alright, so you’re asking this like Goyuu is already in an established relationship, but I could see a couple of possible combinations!

I think the obvious pick is Suguru (given that he’s still around and not a cult leader), since that’s Satoru’s closest friend. I imagine it would be trickier to find a third partner who Satoru is comfortable with rather than Yuuji (since he’s a picky bitch). I read this fic just a bit ago that was about omegaverse bonded Goyuu with an infertile Gojo having Suguru impregnate Yuuji for them, and it was really good! You can read that here if you’re interested: https://archiveofourown.org/works/55263370
Suguru seems the type that’d be more “hands off” (read: distant) with relationships, so I think he’d be a good fit for any lovey-dovey nonsense Goyuu has going on. (also yuuji would love his monster truck cock LMFAO)
For this 3P, I would write it as either: Suguru+Satoru→Yuuji or Suguru+Yuuji→Satoru

I think Yuuji and Suguru both topping Satoru at once could be SUPER yummy and I don’t think I’ve seen anyone else do that before so?? That route would definitely be a “good cop, bad cop” sort of situation, with Yuuji being cute and squirmy as a subtop and Suguru being an absolute shithead to Satoru lol.
But yeah, classic goyuuge with the two of them totally doting on Yuuji with 120% of their attention, full teen strongest duo energy is wonderful and I devour it happily.

I could also see Megumi being a possible third. I think he’d be Yuuji’s pick, since they’re probably the closest to each other, but I think Satoru would complain since he still sees him as a kid (and not in the hot way anymore) LMAO. If we’re talking outside of an established goyuu relationship, I always love situations where Gojo is fucking all of his students (in a casual sense), so I would definitely be down for a threesome with any of them + goyuu.
I’ve seen a couple fics/art where Gojo is getting off watching Yuuji fuck someone, and yeah, that’s also appealing to me. Usually in these it’s Nobara? (since it’s mostly dudebro art lol) but I could see it with any of the other students, too. I love it especially when Yuuji is using all of his physical strength, completely lost in doing what Gojo is commanding him to do.

Oh, also a twins au with Sukuna could also work! Especially if Sukuna is irritated at them fucking in the house all the time, and kinda just invites himself in the group.

head in hands
I hope this answers your question anon!! I love threesomes they’re really amazing! I love that they let you explore dynamics between characters that you don’t always get to see, and I really like how you can balance romantic, sexual, and all sorts of kinds of feelings around. Thank you for the questions, I hope this gives you some brainworms.