Now a days we usually see LOVE triangle situations, they are nothing less than a hell on earth. Whoever does that gets nothing except bad karma.
If you have faced it, just leave both of them with whatever they are upto. You are the legit person, don’t think about that cheap person who betrayed. You will get someone who is a million times beautiful & better than that past. Put your smile, be true to yourself, cleanse your heart, move away & move on.