You could try getting the tires off the ground and see if it shifts into third with no load.
Might tell us a few things.
FYI, no third shift is sometimes low mainline pressure because of worn parts.
Make sure it's full of fluid, and that the fluid isn't dark.
Make sure you have the return spring on the TP linkage that pulls it forward to the stop when no throttle is used, and that the linkage is adjusted at least close.
(travels fully rearward with WOT and comes all the way forward with none)
Pull the pan and find out how many chunks of clutch are in the bottom of it.
That's about the sequence to follow.
