Unknown used transmissions can used transmission problems. Things you get into resurrecting old trucks.
#1 thing to check, does the transmission have lubricant or has it all leaked out?
If I remember correctly the 3 speed can have wear issues with the shift shaft bushings and synchronizers.
It wouldn't hurt to check the previous mentions and external things before going internal.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."