By way of an answer, let me regale you with a story from my own youth, back in the 1900s.

One of my close friends dated this girl for something like two months. They broke up, but stayed friends.

She became interested in me. My friend claimed that he and she were only friends and nothing more, so I figured it would be all right if she and I dated.

After we were firmly established as boyfriend-girlfriend, it became apparent that my friend was still carrying a major torch for this girl. He sank into a deep, bitter, angry depression. Ultimately it destroyed our friendship and rendered much of my relationship with this girl crazy and bad.

I learned two lessons from this:

  1. Never date a friend's ex unless you're willing to lose that friend.
  2. Any love triangle will always involve hurt feelings. Either don't get involved, or resolve that you're going to deal with whatever negative consequences arise from the relationship.

May you escape the hell I went through.