Why does third gear sometimes pop out uncommanded?
The problem you are experiencing is due to a mis-alignment of the blocking rings to the synchronizers. The result is that the pressure due to the helical cut of the gear face forces the shift fork to pop the carrier out of the gear. If you count your shifts slowly, you can get it in gear easily. The result of a popping shifter is from one of two things, a burnt blocking ring or improperly adjusted/aligned aftermarket shifter. Rings become "burnt" or ground when you miss a shift. See below about one of the causes of accelerated transmission wear.