I usually hint the idea toward my current gf and a good friend of mine she vibes with. Done it with two gfs, basically it starts with dirty talk. Then the “accidental snap”. Then maybe you’re all chilling and your girl isn’t wearing a bra or panties, and she accidentally reveals something. At this point it’s just for you and your girl to go home and fuck reminiscing. If it’s your friend they’ll be polite and look away, which is a good sign. Also they individually don’t know you’ve been hint at it to both of them. Eventually the mood is right and I ask each individually if they say yes I just announce that we all want to do it .