There is truth in what they say. If, with only the mid unit running, you get good cold air out of the vents near it, then it's probably running right. It's certain that the ducts that are farther away won't have airflow as strong. It's designed so that when all are running you get good airflow everywhere, but when only one is running it primarily cools the area around its thermostat sensor. The rear A/C is for the rear.