With better AI, UMK3 would be great.
Here are the positive things:
+Violence. Lots of blood, and of course fatalities. It's just fun to blow up your opponent into a bloody mess.
+Graphics. The character models look fantastic and the backgrounds look just as great (espcially the first stage with the skull poles in the lava)
+Sounds. The sounds are pretty horrible (especially the sprint-throw-and pain-grunts, cause they are so long and late) BUT in some funny way, some of the speech are absolutely hilarious. "Choose your destiny" and such.
And then the bad...
-The A.I. This is kinda funny part. Let's just say that the computer's "intelligence" in Mortal Kombat 3 is just so bad. First of all, it predicts your moves perfectly. When you pull a special move, the enemy immediately strikes back (yes, 100% accurately, at the same time). So, if you're using Scorpion, and you use the "spear" attack (Get ouva 'ere!) the enemy jumps forward and kicks you in the air. Second of all, it follows your movement. You will see that the enemy sometimes just stands still. Then you stand still. Then you're both still. Then you start to make quick movements forward (tap the forward button rapidly) and the enemy moves backwards at the same time. Then, once you have moved your opponent in the corner, the enemy immediately starts running towards you and pull a big combo. This ALWAYS happens
-Special moves. If you have skill in this game, you will see that in this game, normal attacks (kicks and punches) are not needed. You can just juggle your opponent in the air the whole round with SPECIAL MOVES. Special move after another you juggle your opponents, leaving them to NOT be able to do anything. Fun, huh?
-Online. Like Gamespot said, the online works really well. BUT there is a problem. Every online player is a PRO MK player so there is no support for newcomers.
Just how bad UMK3 really is? Well, the multiplayer on the same console (not online) is not bad at all, since the fighting system is not total garbage. But, if you try to beat Motaro or Shao Kahn you will hate it.