Once upon a time, there was an arcade game called Mortal Kombat II. It was one of the games that kicked off the 2D fighting-game craze of the early nineties, and in North America, it was really the only competition out there for the almighty Street Fighter 2. MKII was never something that you played for the depths of its gameplay or the intricacies of its fighting system. Instead, it was a vehicle for interactive decapitation, and it was, at least, good at that.
MKII was popular, although it got old quickly if you didn't have a serious taste for grue, and eventually, inevitably, a sequel followed.
Mortal Kombat 3 was sort of a train wreck. It was self-parodying where MKII had been brutal, and added an oddly broken combo system that was based around rushdown. The returning characters were dressed like particularly goofy superheroes, the new characters were silly-looking, and the most popular character in MKII wasn't in the game. Worse yet, the fatalities were, for the most part, almost dangerously lame.
MKII had been a particularly brutal kung-fu movie turned up to eleven (although it certainly had a couple of goofy fatalities and broken characters). MK3, with its cyberninjas, comedy fatalities, cyborg arms, and interdimensional apocalypse plot, was like something that someone would make to parody MKII.
Ultimate Mortal Kombat 3 came along a little while after MK3, to fix some of the problems MK3 had; it, in turn, led to Mortal Kombat Trilogy on the PSOne and N64. It's basically MK3, only moreso; there are seven more characters, including Scorpion, Kitana, and Mileena, and Stryker can now use his submachinegun during a fight, but in most ways, it's a souped-up version of MK3.
As such, it shares most of the problems MK3 has. MK3 introduced the Run button to the series, which allowed a character to dash in and land a painful autocombo. Other 2D fighters allowed for more flexibility and customization with combos, but UMK3 turned the process into rote memorization. Playing the game was a matter of figuring out how best to run in, land your most painful combo, and then get back out. This also meant that only some characters had the tools to really play the game well, which led to a lot of frustration.
UMK3 is also a 2D MK game, which means it's got some of the most frustrating AI in the business. Jade, for example, is ludricrously hard to beat when she shows up in the singleplayer tournament rotation; throw a projectile at her, and she'll automatically get something like a 50% autocombo. No human could pull that kind of thing off, but it's second nature to the MK AI.
Finally, I'll note here that the version of UMK3 on the Marketplace is fairly well-coded for network play, but has a few weird issues. My version has a problem with the horizontal hold, where the extreme left side of the screen shows up on the right. The standard 360 controller's D-pad also isn't much good for this kind of game, as it depends to some extent on rapidly tapping in a certain direction.
A lot of the old arcade titles that show up on the Xbox Live Marketplace are there to cash in on the nostalgia factor, and that's really the only appeal UMK3 has. There are some people who can play this game and instantly flash back to their arcade-rat days, but even then, UMK3 was never very good, and has aged extraordinarily poorly. Unless you're one of the people who were really good at this game back in the day when you didn't know any better, pass this up.