[Excaliber]

Sounds like the synchomesh ring for third is broken. Third gear will still work, the synchro just makes it "smooth" and offers some "resistance" when you engage the gear. If it aint there, shifting would have little resistance as long as the "gear speeds", engine rpm and drive shaft speeds are all in "harmony". If not, it will "grind" a little as the various "speeds" in question come into harmony.

Sometimes I shift my 5 speed without the clutch, just rev the engine to match road speed for the next gear up or down. When you get it "right" it just "falls in" into place with little or no resistance because the "synchro" doesn't have to "match speeds".

Ernie