Hello again Dan. thanks for the tests and the additional background information.
The new units use a much higher pressure refrigerant than the older units, R-22 for the old ones, and R 410a or one of its clones for the ones...these do run very high pressures and very well may be up against their high limits if the water flow is even slightly less than it should be. Your analysis is pretty much correct.
Due to slight variations in how each unit gets water, the #3 unit might be getting just a little bit less.
The other two units are producing 29 and 26 deg F temperature differentials, thats about as close to perfect as you can get...
You will be looking for the same differentials from the #3 unit once it is running again, 29F when it is running in a semi warm room, 26F in a cool room. If you get less of a differential reading, especially by more than 3 or 4 degrees, the unit would have been over charged.
If it runs those 29-26F differentials then it is most likely not getting the cooling water it needs... there may or may not be a valve in the water line that can be adjusted to give that unit more flow... an experienced technician can look that over and tell you what options you have.
Another option is to replace the existing pressure sensor, then run the system, then watch the head pressure and suction pressure
, then very gradually let an ounce or two of the refrigerant out, so that the head pressure drops by 5 pounds per square inch or so, and the suction pressure drops by no more than 2 pounds per square inch. That will make the unit a lot less sensitive to restricted cooling water flows.
That would be the next most recommended step... there are no time limits here, you can have that work done, and be very specific with the technician that you want those pressure readings before and after he adjusts the charge... if he lets too much refrigerant out, the cooling coil will tend to ice... an experienced technician will now just how far he can go in that direction... less experienced technicians tend to run the suction pressure a few pounds higher than necessary... in a case like this with questionable water flow, that could cause the problem.
Let me know how it all goes, I will hold the question open indefinitely, even for months after your positive rating.