VGFacts

In Ryu's ending, where he is facing off against Iron Fist in a fighting tournament, the board in the background states the next match would be Ken versus Mr. X. It is unknown, however, if this is referring to one of the characters called Mister X from Marvel's universe or Mr. X, the T-103 tyrant from Resident Evil 2. None of these characters appear in MvC3 or UMvC3.

T-00 aka Mr.X
Mister X on Marvel Wiki