your third gear dog gear is rounded off
a dog gear is basically a synchronizer, each gear has one between gears, the mesh of the dogs and the forwards gears is very precise
sounds like its been downshifted under rpm without the clutch before
oil will effect clutch slipping but not shifting, clutch adjustment would affect all gears so thats out
as far as a online diagnosis thats what i got, its a pain, pull the motor
split the case and dissasemble, the trans and replace dogs and forward gears below and above the damaged gear to avoid doing it again in6 months
good luck