If that first/reverse gear is chipped, take a close look at the countershaft gear that it meshes with. Also look closely at the splines on the input shaft that mesh with the synchronizer drum. Check that they are still straight and true with no bevel or wear on one side. That is one of the main causes of jumping out of 3rd.
They're not hard to work on, just need a lot of patience and two homemade tools... a thin wrench to remove the input bearing retainer and the tang end of a large file to open the retainer holding the center bearing in place.
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