#1 Provision for any injuries

This may be a damning reality, but injuries can happen at any time. As creatively logical as the other reasons are for making the WrestleMania main event a triple threat, this is just WWE's way of providing for disaster. Any one-on-one match between these three superstars is worthy of main eventing on the Grandest Stage Of Them All, which is always a good thing. The initially scheduled Survivor Series match between Becky Lynch and Ronda Rousey was cancelled after the then SmackDown Women's Champion suffered a concussion just six days before the show.
If any of these three do suffer a similar fate to Becky and are forced to miss WrestleMania, we would still have a main event match between the remaining two women. In other words, WWE is dead set on having the women main event the show under any circumstances. The best case scenario is that Becky, Rousey and Charlotte are all good to go and we have an absolute blockbuster main event between the three of them. If any of them is forced to miss 'Mania, the main event turns into a one-on-one match for the Raw Women's Championship.
Look who just shut down all of Tony Khan's critics! More HERE.
Here's hoping that nobody gets injured and the women put on a killer main event come WrestleMania Sunday!