Charlotte beat Asuka at Starcarde and both Iconics back to back in the previous Smackdown, so you can still argue despite her loses she was the most dominant active female competitor on Smackdown.
So a triple threat with Asuka and Charlotte sounds about right. Plus if Becky is just getting back from an injury a triple threat might be safer for her, in addition to potentially allowing her to drop the belt without actually losing IF they want to protect her in that regard.
The thing is Becky being injured probably changed their plans, and turning Charlotte heel and putting her in that position was probably the easiest most believable way to continue things.