Well, it's not really a throuple. Firstly, because a throuple, or a 'ménage à trois', is a relationship consisting of three people who are in a committed romantic relationship with one another. A relationship where all three people are on equal footing. Since the MC and Chris aren't bi (as far as we know), and aren't involved with one another, by definition the term doesn't fit the relationship.
Secondly, Chris has done made it clear that he doesn't feel that way about her, and it's only a FWB/casual sort of relationship. Maybe this changes in the future, or maybe it doesn't, but it would really be a stretch to call that relationship a throuple regardless of the fact it already doesn't fit the definition.
It's really just an open relationship, and it looks like it could potentially be a two-way open relationship soon, considering Ann is already letting us date Alex.