The only possible (non-MFF) threesomes in the game's future would either be:
A. The MC joins an existing couple for a night (e.g. Gregor (the wolf guy at the lumberyard) and his wife)
or
B. Some kind of MC/F/Futa (e.g. doing Beth and Charlie when the player has chosen Charlie to be futa)
I don't really want to go the swinging/sharing route (at least for the romancable girls in the game) since I personally don't enjoy it, and therefore I doubt I could draw/write it well.