3rd gear grinds!

[1slowdsm]

Okay, I FINALLY got my car back and I guess it was the pivot ball that was keeping the clutch from disengaging. So now that I have that problem taken care of I have yet ANOTHER problem to solve and I need some help on this one as I know nothing about this.
Everything I try to downshift into 3rd gear I get nothing by grinding noise. Once I leave 3rd gear, there is no coming back to it at all unless I go to a complete stop and start all over again. Even with the clutch pressed in ALL the way and the shifter ALL the way in 3rd gear. I emailed the guy who sent me the tranny (TRE) and am asking him the same question. In the mean time I'm hoping to get some information from you guys here so I can get this problem solved. I dont want to end up screwing up my 3rd gear before the tranny is even broken in. Which brings me to ask: How long should I drive to break in the tranny? But back to the 3rd gear thing. Is it the 3rd gear inside the tranny that's messed up or could it possibly be the shifter cables? If anything, I want to avoid taking off the tranny and having to send it back to TRE to have it re-rebuilt and sent back to me.