I remember it's a manga I really wanted to read. I thought it was on my list, but I can't really find it. I thought it was Josei, but I can't find it with advanced search.

Anyway, it's manga about a girl and her boyfriend. They live in a apartment and the girl's ex-boyfriend is her neighbor/lives in the same apartment. I remember the old boyfriend has a long and black hair tied up and the boyfriend has short hair. The three of them are in the cover, the girl and her boyfriend and the ex-boyfriend is like running after them. I don't know.

I want to read it, but I can't find it. I remember it was more than one name and last name ended with "tari" or something? I'm not so sure.

Is it Koguresou Monogatari?

Yes! Thank you very much for helping out. I tried "Love Triangle/s" and "Josei" in my Reading List, but right now I see what I was doing wrong: it wasn't on my Reading List.
-___- Anyway, thank you!