Or you might suspect the clutch disk being warped. Only 3rd gear? Are you giving it enough gas when you release the clutch in 3rd gear? How long have you driven a clutch?
Additional Thought: Based on the giving it enough gas avenue, are your motor and transmission mounts (including the torque stud) up to snuff? If you have slop there it could cause the motor/t-case/x-case to slap back and forth from side to side.