It’s obviously more complex if we’re including Ian in the mix, but let’s look at it with another hypothetical turn of events: Lena and Ian decide to have an open relationship -> Lena and Emma hang out -> some scene that acts like a "gateway drug" to plant the idea in Lena’s mind of Emma as not just a platonic relation (let’s, as an example, say that they discuss Lena’s modelling career, Lena shows her a few pictures, and Emma makes some harmless, but pretty unambiguous comment about how hot Lena looks in them) -> Ian and Lena discuss how they want to “experiment together”, decide they’d be up for a threesome -> they try to think of someone they both feel comfortable with, who they think could be up for some freaky stuff in the bedroom, without any risk of it becoming emotionally messy afterwards -> Emma?
Yeah, that works, but I think it requires more than one chapter to build all that up. Since it falls mostly on Lena's shoulders to first consider Emma as a potential hook-up open enough for a threesome, and then for her to be okay with sharing her with Ian (make sure Emma is not also discreetly crushing on Ian, to avoid the reverse repeat of the Holly drama) in order to mention her to Ian as a candidate. Emma didn't really think about fucking Ian until he made a move on her, so I doubt she'd propose sex to Lena, knowing she's in a serious relationship with Ian now (but not knowing that it's an open one). So there needs to happen a series of conversations that would:
1) Let Emma know the nature of Ian and Lena's relationship (Lena is the one making the move)
2) Lena learning that Emma is an easy hook-up, single and available
2) Emma learning of Ian's interest in her (Lena asking Ian beforehand?)
3) Lena making sure that there are no feelings she's not aware of between Ian and Emma.
Unless you could check all of those boxes in a single conversation that would look natural/plausible, it would require several scenes together. At least 1 with Emma to learn about her sexuality, availability, and lack of commitments, 1 with Ian to see if he'd like for Emma to be a third partner and make sure he doesn't share a history with Emma that would make things awkward between the three of them, and finally the one where she seduces Emma, mentioning that Ian is into her and saying that they're in an open relationship and would like to have their first threesome, and it'd be cool if Emma was that girl. Add to that the fact that most scenes don't happen for one branch only, they need to be there for multiple purposes, so a threesome with Emma is not a primary discussion, but a "by the way" sidetrack from the main event. One event could be hanging out with Emma outside of work/concert which is easy to set-up (Seymour happened, need allies), intimate conversation with Ian could be their bed talk after/before another sex scene (might include discussing all potential partners that they both would be okay sharing each other with), and finally, a Lena x Emma sex scene that happens regardless of the Ian x Lena status (except for Lena being exclusive with Ian). Cramming a threesome in there seems like too much for one Chapter.
Alison is actually among those I find least likely to see in a threesome with Ian and Lena, because it reminds of the whole Axel/Cherry debacle. Alison’s canonically got feelings for Ian, and she’s not good at hiding it, even if Ian is oblivious to it. Even if Alison for some reason would be up for it, I don’t see Lena being interested (as long as she’s in a relationship with Ian—if not it might be slightly more likely).
I was talking in relation to Lena x Robert, so Lena x Robert x Alison threesome where Ian isn't a thing. They don't know each other yet, but Jeremy or Billy might introduce Alison to Lena one of these days, for example if Lena brings her date to Blazer (or visits Mike / Ivy), and Alison is also there with the boys. Dialogue could flow naturally from there with "Hey, Jeremy, who is your friend? I don't think we've met" or "You look familiar... Aren't you a friend of Ian?"