I have no issues with a triple threat. Charlotte vs Ronda was a great match without a clean ending, so I don't see how it wouldn't be fitting for Charlotte to join in on this storyline. On top of that, people are starting to really hate both Flair and Rousey, so they're gonna cheer even louder for Becky, which is all you want after a 7+ hour show. Charlotte's also a great wrestler who's very familiar with both of them, so the match quality wouldn't suffer either.
I was actually rooting for Charlotte to win the Rumble to catch a lot of heat, so that Becky can force herself into the match and not the other way around.. to kind of replicate the whole Daniel Bryan situation where he was the hottest thing going but wasn't booked for shit at Mania, so the crowd rebelled even more in favor of Bryan.
The build up would have definitely been more satisfying if you had the hot prospect hustle her way through, instead of popping the crowd with Becky's Rumble win first, to then try and weasel Charlotte in there while everyone's complaining about it for the next 2 months. Charlotte also hasn't won a PPV match since SummerSlam, so the Rumble would have been her only reasonable way to get to a title match. I'm not sure how they're gonna book Flair into this but even if you're not familiar of WWE's fondness for Charlotte, it still comes across as preferable booking at this point.
So yea, I don't take any issue with the triple threat at all, but moreso the approach to it