eh. itd be pretty disappointing if it was with some character who showed up 60 chapters in. im not against poly stuff, but I am when it hasn't even been remotely hinted at for the majority of the comic.
May I introduce you to Komi-san Wa Komyushou Desu and the trinity of threesome destiny known as Tadano x Komi x Manbagi? The last of whom was introduced 131 chapters into the series.
Well, Manbagi Rumiko has had 100 chapters since, so the developement in her relationships with the silent devine one and the best crossdresser feels natural. Mai-chan here does not have such luxury (yet?), thus even though I am an advocate of polyamory, for now I agree with runrin that the poly route would not be a good choice, for it would feel shoehorned. Not without about 2 more volumes for developements.