You may as well tell. You waited too long as it is and the longer you wait, the more serious the other two become. Maybe you can stir up the emotions a little bit and at least make her think hard about one or the other instead of just forgetting all about you. And if she decides to stay with him, well, that's too bad.

I mean, you really shoulda done something much sooner, but it's too late for that. Don't wait any longer.

Alexi

PS - And don't tell her through email or over the phone. When you do it, do it in person. She'll take you much more seriously than if she just reads it.