All the gars use the layshaft except 4th does drive engage in 4th ? if so itÂ’s a layshaft problem or your synchromesh assy are broken, bad news whatever.
Does it select 3rd at all or just wont go in? Use this link to download a manual PDE book 4 page 29 shows the suspect item listed under the Layshaft column as No 8 & 16