Placental mammals (like us) basically start with 3 precursor tubes that fuse together early in development.

In marsupials, the ureters (which transport urine from kidneys to the bladder) pass in between the 3 vaginas, so it would be pretty much impossible for them to fuse without cutting off the transport of urine to the bladder. In placental mammals, the ureters develop differently and no longer pass between the tubes, which is why it's possible for us to fuse the 3 precursor tubes into one.