89 toyota 3rd gear very bad slipping?

hi guys i have a 89 toyota cressida with an atomatic and when i was coming home from work one day im driving then as soon as the shifts to 3rd gear it felt as if im in neutral it was not moving the car at all in 3rd gear, but first and second work fine, the is pretty old to i have been planning on rebuilding it but i would like to pinpoint the problem before i do anything. i will appriciate any comments please thank you