if all other gears ok then selector fork possibly worn or more likely syncromesh in the third gear worn, The syncromesh in side the gear has to stop the gear turning while it engages the other gear. suppose you don't know how to double the clutch never to late to learn
wirecutterman
February 2008
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.