Hmm, I think something is wrong with your torque converter. It's possible that the overrevving clutch is worn. I could be wrong though but what happens when it is worn is that, it takes longer for you to accelrate because it isn't grabbing onto the turbine. The turbine in the torque converter is what turns the input shaft. When you're in 3rd gear, what rpm are you at? Which clutch did the shop say was bad? If it is your torque conveter, you can fix the torque conveter if it's bolted together, if it's welded together, you just have to replace it.

Older automatic trannies have torque converters. Newer ones don't necessarily have them.