DNA test/child support

It's a very long story, I'll try to make it short. My fiancé used to date a girl and possibly had a child with her. They broke up and she started dating another guy BUT she had intercourse with my fiancé and another guy (her new boyfriend) in the same day and that is the time she conceived. Sounds like a movie I know lol. The child is now almost 6 years old and my fiancé still doesn't know 100%.. everyone that knows my fiancé, this girl and her other "baby daddy" all say that the child looks so much like all 3 of them and I agree. She had told my fiancé in the past she had a DNA test done but never showed him proof. The other guy has always questioned it too. A year ago my fiancé brought it up to her again when she was going through a lot of issues with DHS because if the child was his, he wanted to take it from her. (There was a whole incident with her newest boyfriend beating the child) anyway, she refused to show him a test and blocked him. At that time, dealing with DHS, they had to do a DNA test for child support from the other possible dad (the one who has co parented the child with her)