need help wont shift into third

the shop said that there could be a crack of some sort in the converter that wasnt seen. they open up the valve body and it was clean. filter clean, just all around everything is clean. so bad converter. I went to a local shop and got a hughes 2500 stall converter for 294 being that the shop said replace the converter it will solve ur problem. as for the gears, they said as it leaks out the converter u lose pressure from loss of fluid so it wont shift all that great, makes sense.