With three families there are too many variables to go wrong if one person tries to coordinate all FP+s and ADR's. There are all sorts of subtleties like an adult must make a FP+ when a child is too young to ride/experience alone. You are likely to run into all kinds of problems. The system isn't so friendly to multiple emails, multiple rooms and multiple credit cards.
I would never link three accounts in
MDE. It would be too easy for someone to wipe another families' FP+ reservations.