Depending on what you mean by "meaning", all three would translate naturally to:
Where is Mary?
But the difference is the degree of politeness.
でしょうか
is a polite way of asking questions. As indicated in a different question on this site, What's the difference between でしょうか and ですか at the end of a question?, でしょうか can be used to ask a question that expresses uncertainty. でしょうか is the polite version of だろうか.
So it's not quite the same in literal meaning as ですか and cannot be used in every context where ですか is used.
いますか for 敬語 changes to いらっしゃいますか or perhaps better いらっしゃいませんか
ですか for some 敬語 changes to でしょうか
マリーどこ? < マリーさんはどこですか。 < マリーさんはどこにいますか。 < マリーさんはどこでしょうか。 < マリー様はどちらにいらっしゃいますでしょうか
And for the more polite ones, depending on relationship you might want to use [様]{さま} instead of さん