Considering what we saw of Axel and his reactions to Lena, I don't think it's likely he'd involve her in a threesome with another guy. He seems to want her all to himself and only willing to share her with another girl.
You're right, but maybe in this threesome scenario, it's not about Axel getting what he wants, but Lena getting what she wants for a change.
I mean, we know Axel wants Lena back, and that he's unlikely to get her back as he
currently is unless she goes down the corruption path and is up for anything (in which case she probably wouldn't be exclusive to Axel anyway, so he could take it or leave it, and in this case he'd be taking it); or an Axel who is in love with Lena cleans up his act in some way so Lena will reconsider him as relationship-material. In the latter case, it wouldn't be unreasonable for a Lena who's become more assertive (sick of being the 'silly naive girl who was cheated on) to have her say in the sort of sex the have, and we know she's not against a threesome anyway, since she was okay with going along with the whole Cherry arrangement as long as she thought Cherry was just 'some girl', and not fucking Axel on the sly. So maybe she'd be open to threesomes in the future providing they're transparent about it - no more lying - and she gets to choose the third party sometimes too.
Anyway, that's just how I saw it potentially working if there was gonna be a Seymour/Lena/Axel threesome, and providing the player was going to get a choice about how much control Lena has over her life and her relationships (if they want it that way).
EDIT: That being said, you're right in that it would be hard imagining Seymour being willing to share Lena in any scenario, at least where he wasn't the one pulling the strings in the situation. Can't really fault that logic.
Axel and Seymour is a very interesting duo. They both want to corrupt Lena for their own selfish reasons. Although now they are working together (even though I suspect Seymour is using Axel without the latter being aware of it), it might end up leading to conflict betweem the 2 down the line, which could potentially lead to a very interesting scenario.
That does sound pretty interesting, too.