[CarPsyco84]

I would double check that your clutch is not dragging, cause if your 3rd gear syncro was weaker than the rest that would be the first gear you would notice it, if the clutch is working fine, then the syncro is worn out, if you keep grinding it, the shift hub and 3rd gear will start to wear and it will not want to go into 3rd anymore. You could try redline mtl/mt90 if you were not already using them, sometimes they make poor shifting transmissions shift better, but don't expect miracles.