Othello - Miku's friend Sumire confesses to her, but she rejects her because she likes guys. Well Sumire wants nothing to do with her if she can't be with her and that causes Miku to realize she likes Sumire.

Kimi Koi Limit

Though... The main girl is the one who ends up with the 3rd girl at first, and then gets a chance with the girl she originally loved.