The fact that the transmission appears to shift well in the other gears would tell me that the problem is isolated to third gear, most likely the blocker ring. Too much lubricity often times reduces the requisite friction needed for the blocker ring to slow down the cone clutch on the corresponding gear. That is why
synthetic transmission fluid is not suggested on a Toploader.
My suggestion would be to run it and see if the ring seats in over a period of time. Just shift it easy so you don't chew up the engagement teeth on the gear or the synchro slider.