Ultimate Mortal Kombat 3/Walkthrough

First Battle
Any kombatant will work as long as you can avoid the computer's kombos preceded by him or her running towards you. Your offense should begin with a flip punch and any kombo that starts with using high punch. Keep repeating this on your first opponent, and the battle will be easy.

Second and Further Battles
At the second battle and onwards, the best kombatant to choose to destroy the computer is Noob Saibot or someone with a Spear like Scorpion or both of the Smokes.

Noob Saibot is probably the best by far due to the creativity you can add to his special moves. You can start the battle with either a Shadow Throw or Teleport Slam. If using the Shadow Throw and after its success, you can move where your opponent is about to fall and hit him or her with an uppercut. Your next move sequence should be a sweep, Disabler, a kombo or two, another sweep and Disabler. If you resorted to using his Teleport Slam, you can proceed it with an uppercut or Disabler, and follow the same sequence. If you miss your slam, immediately throw a Disabler or shadow. Noob's Teleport Slam and Shadow Throw also are good ways to snag an escaping opponent, and you can always finish those two moves with an uppercut.

If you've chosen Smoke and to catch the computer off guard, try executing a backflip and then a mid-air Smokeycut. After a successful Smokeycut, deliver a Spear and a good button-link kombo. Also of note, a flip kick followed up with his Smokeycut succeeded by a Spear and a good kombo is lethal. To completely thwart the computer's plans, do a backflip into a Smokeycut, Spear, any good kombo, and follow the kombo up with a sweep to force the computer to do a backflip. Afterwards, use the Spear, any kombo and a sweep. Repeat the Spear, kombo, and sweep.

With Classic Smoke or Scorpion, the strategy is different. Try the flip-punch combo as mentioned in the first battle, which starts with a flip punch and any kombo that starts with using high punch. Afterwards, follow it up with a sweep and the Spear to catch the opponent backflipping. After a successful Spear, do a good kombo, sweep, and repeat.

Fighting Smoke/Sektor
Smoke and Sektor both possess the Teleport Uppercut, so flip at them so that they use it, and you can simply uppercut them, or use the aforementioned Smoke kombo. If you're Kabal, however, you can use his Raging Flash followed by an uppercut, or jump punch and mid-air Gas Blast.

Motaro
Avoid getting too close to Motaro, and never throw projectiles at him since they reflect at you! The best kombatant to use against him is Kabal, Scorpion, or Classic Smoke. Their high-punch kombos stick to Motaro, and you won't come up short-handed and miss. All you have to do is begin with a flip punch and quickly execute any kombo that begins with high punch.

Shao Kahn
Beware of Shao Kahn and his Sledgehammer! Start the match crouching until Shao Kahn does his Upwards Shadow Charge. Once he's airborne, quickly uppercut him, and run towards him and execute your best kombo and a projectile. You can crouch again, and wait for his Upwards Shadow Charge so you can perform the counter again.

If you're Smoke, you can catch Kahn with the Smokeycut and another Smokeycut, Spear, and a kombo of your choice. Take advantage of the double Smokeycut since Kahn is the only character you can use it on.

With Kabal, you can wait for Kahn's Upwards Shadow Charge, uppercut him, run at him, press Low Kick, Low Kick, High Punch, High Punch, Down + High Punch to send him airborne, do a jump punch with a mid-air Gas Blast, and do another Gas Blast when you land. Return to the crouching position and repeat. If you really want to be technical, you can do the Raging Flash before uppercutting Kahn and the rest of the kombo after Kahn's Upwards Shadow Charge.

Using Noob, you can decimate Kahn with the Disabler. You can implement the same strategies, but keep in mind that the Teleport Slam is completely useless, and you only have time to execute one or two kombos during the Disabler's effect. Shao Kahn is no more!