I tend think REN is rather irrelevant these days. I would of thought it was bloody hard to find a handset that is 1 REN.
These days people at home are just buying Uniden/Panasonic DECT sets where just the base unit connected to the line and the rest are DECT.
for a single handset I completely agree. But the OP said they had 3 units attached to the one common line.... and without knowing what physical kit is connected, REN becomes very important. I once had a customer who had an entire outdoor paging system for their goods yard, attached to horn speakers and triggering visual lights to indicate a call. the calculated REN was 4... and that only became clear once we migrated them from a POTS port to one supplied from a PATTON unit.
Anyway I'm sure the OP will have things all in hand.... or failing that our resident analogue guy Mr Biddle can help, when he finishes his speaking tours of NBR.