In order to determine if 3 people have a common ancestor, you need to have the following two requirements:
1. All 3 of them must match each other on the same part of the same chromosome
2. All 3 of them must be in common with each other
I usually get the most responses from my ftDNA Family Finder matches shortly after I've assigned all of my matches a known relationship. You know, that little orange button that says "assign" beside all of your match names?