With the recent interest in UMK3, I decided to make this thread. First of all, it would benefit you the most by visiting the sites located in my sig. (www.UltimateMK.com / www.ClassicMK.com / www.MortalKombatEmpires.com) A great majority of the best UMK3 players are on these sites, as well as any information you could possibly need about UMK3.
UMK3 can be played online on XBLA, MAME/Kaillera/P2P, and the Nintendo DS. Whichever you play on is your choice, though there is some differences in the DS version of the game. And you can no longer download UMK3 on XBLA, since Midway is no longer affiliated with MK. So presumably the best option would be on Kaillera, which is free anyway. Visit the aforementioned sites to learn how to accomplish this.
There is several upcoming events that will be holding UMK3 tournaments. Such as on June 11-13 in NYC, Road to EVO 2. UMK3 will be streamed at this event by Team Spooky, so be sure if you can’t attend the tournament to at least visit the stream. All the information about the tournament in general can be located here: http://shoryuken.com/showthread.php?t=222564
This thread can also be a discussion thread for UMK3. And any questions you might have, you can ask them here and I will try to answer them asap. I realize this forum is not in love with MK games, but if you dislike UMK3, just don’t visit this thread.
Now, a few things about UMK3.
UMK3 has a very small top tier, a huge mid tier, and a small bottom tier. There are only a mere handful of universal infinites and almost all characters have counter characters who are traditionally ranked very differently otherwise and might not stand a chance against certain ones in between. All exploits other than the 3 or so universal infinites in the game, are so limited or difficult that they don’t break the game. UMK3 is a fairly balanced game, this is a misconception due to there always being a Human Smoke or Kabal in the finals. While they are the best two characters in the game, as was said above, every character can compete. And UMK3 just doesn’t get the numbers as most games, so the variety in character choice is limited.
UMK3 has a lot to it: Ground Combos, Set-ups, Positioning, Spacing, Footsies, Blocking, Run, Run Jabs, Kara Jabs, Corner Runjabs, Throw Defense, Crossups, Crossup Throws, Special Moves, Punishers, Anti Air, Damage Protection, Pushback, Corner Resets, Fake Ducking, Wake Ups, Glitch Canceling, Relaunch Combos, Buffering/Block Buffering and Off The Grounds. Mind Games play a huge role in UMK3 too. Most of these things are known in other fighters as well.
Most if not all of this is covered in the guides on UltimateMK.com. There is also character specific guides there as well, which are very detailed might I add.
Again, any questions you have, please post them! 
For match videos from various players, visit this youtube channel: http://www.youtube.com/user/TheUltimateMK
Button set-up for MAME: Thanks to snaek
button 1 = hp
button 2 = bk
button 3 = hk
button 4 = lp
button 5 = lk
button 6 = rn
so it looks like this:
1 3
2
4 5
6
or if you prefer a standard-style 6 button layout:
1 6 3
4 2 5
