If its a stick shift it could be synchro(s). when shifting from 2nd to third put the clutch in, pull it out of gear, clutch in, clutch out, clutch in, shift. if it goes the synchro's need servicing but that would likely mean rebuilt tranny or cheaper, junk yard.
If its an auto theres something broke that you cant fix and you need a real mechanic. AAMCO will give you a free estimate