I will share what I recall to be the most enjoyable positions and activities in my threesomes. Overall, though, I’d say the set of combinations of who’s mouth and genitals are on/in whose others’ mouths and genitals is fairly intuitive, and one should try them all!
But to break them down:
Going at one girl from behind while she licks the other’s pussy is the standard go to, and both comfortable and good fun for everyone involved. Note that you will need a fairly large bed or other surface for this, however. If your bed is not that big, you can also stand off the side of the bed and then enter girl #1.
You can also be fucking one while on top of her while the second girl sits on her face, with the added bonus that you can now run your hands on the second girl’s body while you are going at the first one.
Lying down with one girl riding me and one on my face is also quite delightful.
At some point during your session, it is definitely compulsory to have both girls go down on you (one licking the balls and one sucking the cock), this is of course tremendously pleasureable.
A third person also opens the door for simultaneous SM play, for instance one girl whipping both me and the other girl as I’m banging her. Or she can spank her while she is sucking my cock, I can mouthfuck and slap one girl while she is being licked, etc.