For a first threesome, I think it'd have to be someone Ian and Lena both know semi-well.
So I think the most likely options for MFF is Emma, Ivy, and Holly. MFF will probably happen before MMF. For MMF, I think Jeremy is the most likely option in a later chapter, unless Ian and Mike get better acquainted.
Random thought, I really liked playing Ashley in GGGB as an ethical slut, but it's difficult to play Lena this way (unless you're focused on the lesbian side). Robert feels too manipulative IMO, Mike (would be) a cheater, and Jeremy is iffy at best. It's actually easier to play an ethical rake with Ian, you can get with Lena, Allison, and Cherry without being too much of a dick.