Does the tranny mount cause any extra vibrations and/or put any additional stress on the drivetrain or other mounts?
The cause of the 3rd gear mis-shift/grind is the stock rear motor/tranny mount. The stock mount allows the engine/tranny to sway during WOT and stretches the shifting cables that preevents 3rd gear engagements; along with 2nd gear popouts, etc.
Just by simply replacing the rear motor/tranny mount with a TRZ Poly or similar, you will have a perfect shifting tranny at any load...
Upgrading the fluid will only make the tranny smooth...the shifter bushings will not help with 3rd gear grind.