93 probe gt transmission problem

i have a 93 probe gt v-6 with a 5 speed trans. when i stop and go into first if i dont let the clutch out slowley it will grind the gear but when i let it out slowley it jerks itself into gear which i guess is better than grinding the shit out of it, but once i let it out and it pops into gear its fine until i stop again and go back into first. also i have no 3rd gear at all all it does when i put it into 3rd is grind and it wont go into gear at all. all of the rest of the gears work just fine. the previous owner said it started doing that after they swapped the motor. he said they just swapped it out for the same year motor with about 70,000 less miles on it then the old one. ever since then it has been messed up. can anbody help its driving me insane:banghead: