Given that we know that we will have to choose our special LI one day, our hero already had a situation with a throuple gone bad, it is pretty much clear that any kind of throuple ending(!) will be in a bad ending.
As for the threesome Feli/Mina/MC I consider it "wrong" in the meaning that for it to happen, you loose big chances for both Felicia AND Mina romances. It is a nice threesome but nothing more, hence fuckboy stuff and you loose more in the process than you win.
A threesome Hana/Mina/MC will most probably come and be hot, but still "wrong" on the relationship end of the equation. To even get to the initiate stage of it, we see jealousy rearing it´s head. Yes, a nice fun threesome or two, but a throuple MC/Hana/Mina will never last.
So a throuple ending by design will only happen on a bad corrupted ending, because of the characters involved. e.g. Hana has nothing against having some fun and has an understanding as far as the club in involved. But beyond that, she wants an exclusive relationship. So some threesomes, esp. fun ones yes, but not longterm stuff.