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 Smoke. 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. Smoke wins!

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! The best kombatant to use against him is Kabal since Kabal's high-punch kombos with his hookswords will stick to Motaro. All you have to do is begin with a flip punch and quickly execute High Punch, High Punch, Down + High Punch.

Shao Kahn
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 Smokecut, 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 following the rest of the kombo after Kahn's Upwards Shadow Charge. Shao Kahn is no more!