There are three clutches operating in third gear, the rear clutch in the main trans, or "forward" clutch, and the direct clutch in both the main trans and the OD section. If I had to guess which one was slipping, it would be a difficult choice.
If the slipping seems to happen in all forward gears, its the rear clutch in the main trans. If reverse is affected, its the front (direct) clutch in the main trans. If its only third gear, its possible the direct clutch in the OD is slipping.
Beyond that, its not impossible for something electronic to be failing intermittantly, and not caught by the PCM yet.