UMK3 tries to improve MK3 by bringing back MKII characters. Too little too late.
Nothing about MK3 is improved with UMK3 other than the new favorite characters. Game play is the same, graphics are the same, and sound effects are the same… but a few extra backgrounds that don't really do much for anybody besides give them a reason to release another game to get a few extra dollars. Two new characters are introduced: Ermac and Rain. They're moves are great and useful… but that's about it.
The SNES version is lucky enough to have all characters (aside Kahn and Motaro) initially playable characters. However, many of them are missing fatalities and friendships. This shows you how lazy they were with this game. Brutalities (a "combo-ality" if you will) is introduced as a new finisher, but it's nothing worth talking about in depth.