missing 3rd...

Here's a wild guess;
insufficient clutch departure, unless it also grinds while attempting; then; wrong oil, or bad blocker rings.

But if an A833, then since you say; always and only third gear, then linkage adjustment; which includes a loose 3-4 lever retaining nut. Those buggers like to come loose and make trouble, especially going thru the 2-3 gate.