What rpm are you trying to shift into 3rd?
Does it shift into 3rd OK at say 5000rpm and lower? crunch at 6500rpm or higher?
Next time you drive, give the stick a push toward 3rd gear and see what rpm it drops into 3rd gear without crunching.
Could be just normal synchro wear, or you're trying to shift at too high an rpm for the single synchro setup.