Out of the 7 couples I really don't see any of them making that sort of leap into threesome territory. With the exception of Isabel and Alma, the girls don't even masturbate, whether it's purely for the camera or not.
It wouldn't even make sense for Leora/Paul to have one since be can barely last more than a few minutes, and them along with Max/Lora, Isabella/Marcelo and Nora/Kiko hardly ever have company (not to mention that Kiko/Nora only have sex once every 67 days). Alma doesn't seem secure enough to me to share Stefan with another girl, and Isabel comes off as too possessive of Marcelo.
That leaves Maya and Alina. Maya MAYBE would go along with it if she had enough alcohol. I still don't really see it with her though.
I'm not trying to sound biased since they're the only couple I really pay attention to, but I think Alina/Anton out of the rest of them would be most likely. I think she's a much bigger freak than the other 6 girls and she comes off as the most sexually adventurous. The only problem with them is their company. The girl (sister?) that comes by all the time is definitely not into joining them and other two girls VERY seldom visit, and one of them is just as reserved about the cameras as "the sister."