There's Werewolf threesome drawn when you ask in Brothel to get owned by them.
There's a threesome with Catgirl × Werewolves, but it's essentially the same reused art and can be a foursome too.
There's a threesome with faun and lizard girl when you get a little drunk in the tavern and then they can take you on the table. Has art.
There's a threesome with Harpies, has art. It's a Mating Grounds type encounter.
I think Slime technically can give you a threesome with her copies.
There's a threesome of Trudy with goblins, has art. You need to meet Selkie once, next time you take two different paths with Trudy.
There's a threesome of three of your companions if you book in the Silajam inn. First you sleep alone. Then you see Trudy getting naighty but walk in. Next they all get dirty. It has art.
There are other group scenarios that don't match criteria, like with Angel and her bro (no art). There is a foursome bukkake if you lose to Jester in Story Mode, has art. There's a group violation for losing to Ghost, has art. Same for failing to steal, has art. So yeah. A lot of stuff.
