good question, I think that Mary Poppins might add a little lift to her ride, but with Liberace it really depends upon the length of his cape trailor and the amount of diamonds it contains. Though the diamonds may add a bit of weight, they have the potential to blind his opponents.
I would bet on either Liberace or mister Rogers being the superior combatants based on their strong attraction to the 6 o'clock position. Trying to shake them would prove futile.
Although, I'm not really sure what Poppins would do if there were children involved. She has alot of potential there that could make her Mossie more nimble than usual. She also has that umbrella that may give sustained flight and keep her from going down on them while they pummel her tailpipe, (as if that would ever happen)
In a 3 way, I see liberace coming out on top of Mr.Rogers, giving Poppins just enough time for an unwelcome low blow on Rogers, taking him out early but leaving her vulnerable to Liberace (who may or may not be able to hit his target due to a lack of interest).