Don't fight the computer like you would an actual player, you need to learn their "logic"
For instance the first few matches you can always hit people with stun moves (cyrax's Net, Zero's ice, Scorpion's Spear, etc). If the AI isnt moving and you run towards them and they still don't move you can usually get a combo in. If they start to move away from you when you run in they'll usually get a cobom off on you. Kitana/Mileena/Jade can be hard to uppercut if you get a stun move in the air and you can't do the jumping HP to start a combo on them either.
What I do is use Cyrax and use his net, once I get to the point where the AI starts blocking or countering it I turtle up by holding block and standing there tapping away and just letting the net got. They'll usually get stuck in it about 1/4 of the time. It takes longer and sucks this way but it works.
Beating Motaro with cyrax is kind of easy too, just jump away and if he jumps jump kick him. When he falls run and do the 6-hit combo (he'll usually block but you sometimes get all of it or part of it), jump away and repeat
Beating Kahn is kind of iffy, you have to get lucky and hope you get a net on him without him returning any hits (fireball usually) What you do is just keep launching the nets, he'll usually hit you with his fireballs but as long as you get at least one clean net on him you'll outlast him by doing Cyrax's 7-hit (jump in hitting a HP in air then the standard 6-hit combo).