I actually think a triple threat makes a wild amount of sense, but they're not going about it organically, which is irritating. The wrestlers should have their own agency instead of the McMahons just doing it "because reasons"
Build it organically. It's the culmination of a year of storytelling by WWE. Rhonda comes in, avoids Flair, so Flair has beef with her, Rhonda and Flair's only match ends in essentially a no decision, they're not done with each other, Becky and Flair have had issues going back to Summerslam (or earlier), and their blow off match got interrupted by Rhonda, so Becky has beef with Rhonda. It's one of the few extremely natural to build itself Triple Threats I can think of, and it would also help take some of the weight off Rousey both in-ring and from the fans. In-ring do you really want the first time a Women's match is ever going to HEADLINE WrestleMania to be a woman who's wrestled a handful of matches and is possibly leaving soon? No, you want it to be the two pillars of your show. The ones who have gotten you to this point and will carry you beyond. And you also need to give casuals something. AND if this is going to be Rhonda's farewell match, even if it's just for a little while so she can be with her family, you want her to go out with the crowd at least somewhat amicable and you don't want a repeat of Lesnar-Goldberg. So it all makes sense, but just...do it in the ring. Have Flair screw with Rhonda in a match and so Rhonda asks for a Triple Threat, or Flair screw Becky out of something so she adds Flair, or Flair just do...SOMETHING that allows her to put herself into the match - Hell, throw into question the validity of Becky's Rumble win since she wasn't in the match and added herself...then she and Flair fight to a bunch of no contests so it's a triple threat....SOMETHING. There's a dozen ways to do it....Having Vince just shuffle things around because "random villainy" is the laziest one.
It's the right move, but the wrong way to get there.