every time i change to third gear it make like a grinding noise like im not down enough on the clutch but only in third gear and now reverse, i have been told either slave cluctch cylinder, third gear selector could anyone give me advice please

I'd check the condition of the "dogbone" engine mount, and the solidity of the "console bush" first.

If those are ok, check the gear linkage adjustment.

Next check the slave cylinder (I don't think this is likely, or your problem would show up worst in first and reverse).

Finally, it means investigating the gearbox internals (if 3rd and reverse are on the same layshaft your symptoms make sense, but this is expensive, hence doing all the cheaper checks first).