Those are certainly the logical choices
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and mouthwatering ones at that and if I remember a couple of updates back Ellen was curious about Victoria in a conversation she had with our MC
That convo happened indeed, and it set a variable if you chose the right option.
The most logical and obvious one seems to be Chandra and Abby, so much so that honestly I would be surprised it that didn't happen. It seems almost inevitable. Also, some kind of teasing involving Meredith might be a possibility; I mean, Meredith watching the MC fucking Chandra or something along those lines. But that's just a possibility, I wouldn't take that for granted at all.
For me, Ellen and Victoria looks like a very logical 3-some too. Obviously, it would all depend on our chosen path. If we're on the spellbound route Victoria will continue behaving like a bitch, the MC will be a disloyal asshole, and I don't think Ellen would be too happy to be in the middle. But on Victoria's good route all evidence points in that direction. We've read Ellen imply she gets wet thinking of Victoria and asking about details of the MC's sexual encounters with her.
One threesome that I don't think will ever happen? Ellen and Gloria. Ellen is too protective of Gloria, who's like a little sister or goddaughter or whatever to her. I don't believe she'd be happy to learn the MC has had any sort of sexual relationship with her little 18-y-o Gloria.
Katie on the other hand will gradually open up to the MC. She'll never be a slut, that would be totally out of character, but if you chose to ask her to be spicier (which you can during certain sex scenes and, again, it changes a variable in the script) she'll most likely start to become bolder and to look for new ways to please her man. Let's assume that in a couple updates Gloria and the MC can finally get closer and become intimate, with Gloria developing true feelings towards him. On a good route wit Katie I can definitely envisage a scenario where she makes it possible: "A nice girl like Gloria loves the MC, I (Katie) love the MC, the MC is a trustworthy person who loves me too... OK babe, let's do this."
I'm not interested in any sexual intercourse involving Kleo, so I'll leave the discussion regarding a 3-some possibility with Sonja to the others who want that. In any case, I find it improbable.