I had the same problem where 3rd would grind at high rev shifts..
I figured to change over the tranny fluid to 20/50 and it still did the same thing until after driving it for a couple weeks - then it cleared w/out any more difficulty.
I also had the same problem with 5th (at any rpm).. which also got better after the switch and driving it for a while.. yet still does w/ higher rpm quick shifting..
my .02
seeya -
Jason