3rd gear synchro. Depending on what kind of car you have, it might help when you're going to third(either up or down) to:

1. shift to nuetral, let out clutch
2. put in the clutch, and slowly ease in to third.

In the meantime investigate how much work is involved in swapping out the synchro. Also, if you have a short shift kit try to locate the original linkage/cable to revert it to stock.