My transmission is slipping/not shifting to 3rd gear

First thing to do is always check fluid levels... get car warmed up, then with feet on the brake pedal run it through all the shift positions from park to first...now put in park,leave car running and check the fluid level it shouldnt smell burnt and if low add a small amounts at a time till its brought up to proper level. start with half pints at a time..this isn't like adding car oil.
if this is good then its time to let a transmission shop have a look.