If you take your trans out of OD and into 3rd gear, I don't see how there is so much heat generated the tranny anyway. It's just in 3rd gear, transferring energy thru it's gears to the engine. The engine is doing all the energy transfer, not the tranny. The transfer thru 3rd gear is probably 97% efficient. I highly doubt any significant damage is happening in the tranny in this case.