Car will not go into reverse or drive

Automatic transmission. I got the car given to me. In the beginning it would be placed in reverse but only backed into reverse after about sixty seconds of gear same as in drive in the beginning. So my coolant light was on but no overheating at all but once driving on the road it would function at normal selected speed. So one morning after returning from a road trip it stalled more than sixty seconds in reverse it was about fifteen minutes of stalling before going into reverse. Then on the next day it would go into rev and drive but no movement at all. I got it towed to a shop that new how to drain the transmission fluid out of the tank but not much knowledge on transmissions or Saturn's. But only put three [quarts] of transmission/fluid back in the hole where the dipstick was removed. After about three times of switching gears while the car was on the lift the tires started moving In drive and reverse but stopped. He then said it was the filter clogged and that the filter is inside the transmission so the transmission had to be replaced because there is no filter cover to remove. He then pulled down the car from the lift and the car drove me home where I stopped for gas and reversed it in my driveway then it stopped again. Please help pin point the problem he said there was a code that said gear or clutch involved but he said that meant transmission. The car would move a little in the drive way then sop and sometimes not at all. Just depends on how consistent I am in checking.