Bad vibrations in 3rd gear

broken third gear.....


Thats my guess. If only in third and it doesn't matter what rpm, then there is something either wrong with the gear or cluster.

All the gears in a manual transmission are always rotating, regardless of what gear you are driving in. So if you really chunked some of the teeth out of a particular gear, you'd feel it at all speeds in any gear.

Like I always say, with ANY transmission, its really impossible to completely diagnose without tearing it apart, so it sounds like you have some work to do. Pop the cover off of it and have a look inside. See if anything appears broken, loose, or misaligned. Transmission work is pretty involved, so if you're not confident with it, take it to a shop.