my r reg 306 it fine in ever gear apart from 3rd it makes a cruch sound

Sounds like the gear box is about to give up.
If that is the case then it wo not be worth repairing on a car that old.

The syncromesh is just worn on 3rd gear, if it does not get any worse then do not worry about it.

The syncromesh is weak on third gear thats all. It is not seriousbut if it is really bothering you then you can ge it repaired. But it is not the easiest job in the world so it wont be cheap.

Yes -sounds like the synchro gear is worn or non existent, - try being 'rev' sensitive and double-declutching when changing up or down.
May extend the life of the box a little.

Best wishes.