I had a great running tranny and one day 3rd started popping out for no reason. It got worse over the day and I finally pulled the side cover off.
I had an exploded synchro ring (see picture). Some small pieces had messed with the linkage inside the tranny. Picture of the metal pieces

Once everything was cleaned out, it still works. It needs to be taken apart and repaired but I can drive it a bit until getting fixed.

get that side cover off and make sure everything is intact. Make sure it's in neutral when you take it off and put it back together.

Cheers