well more than likely what is happening, is the slider and/or synchro hub is the problem. the slider is what the shift fork moves when changing gears. they have little teeth on them shaped like little dog houses. as they wear out the little dog houses become worn and no longer hold solidly into the gear selected. it is also possible that the detent spring for 3rd gear may be worn out. of the many manual trannys i have built this seems to be common. the cause is usually from either grinding the gear or not engaging it all the way.
-----------------------------------------------------------------------1970 SIIa 2.25 petrol 88
1970 Series IIA 88" Petrol