#2 Ronda Rousey interferes to beat down Becky again
One of the biggest reasons why this has to be the case is WWE needs to guard the third entity of the triple threat match. Charlotte has already been dominated by Becky for a long time and even lost to her again at the Royal Rumble where she was tossed off the ring.
Putting her in a match where the only stipulation for Becky to qualify for the match at WrestleMania is she has to beat Charlotte, really puts the Queen in a tight spot.
A way out could be Ronda again taking things in her hands, and in a quest to add Becky into the match, she interferes, attacking Becky yet again, which causes a disqualification and automatically adds Becky to the match.
We can't have either of the women lose the match cleanly, so this has to be one of the ways all three go into WrestleMania looking strong.