Out of the three, Celine has managed to take care of her voice the best...
Mariah has mostly good vocal moments, but as a fan, I admit that she has bad moments, but her voice has been improving in the past year. Plus she has a new management, which also manages Jay-Z which is good compared to how crappy her previous team was...
All Mariah needs is rest.
Whitney unfortunatley ruined her voice due to drugs
It wasn't the same when she had her comeback album (RIP)