right, so what you need to do is throw out the model of cladistics - the "tree of life". the model keeps shifting, but it's unworkable. you need to think of it more like a complex graph of genetic interplay...

there would have likely been a handful of related primate species around a few million years ago that could have interbred with each other. so, you would have had the genes flowing around in a complicated web of flow that went through various transitional stages and ended with the three species.

there is not one common ancestor that the three species shares. there are several, and they've left us their genes in a complicated, non-linear way.

http://www.upf.edu/enoticies/en/1112/0309.html#.VEjnCBCGrZ4

this'll get them going: in a crude sense, it is probably true that a distant human ancestor was created through the hybridization of gorilla and chimpanzee ancestors.