Is the power brick working? Do all three chairs share the same power brick?

Have you checked all the wire connections underneath the seating? Given the age of these chairs, I would bet that some of the connectors have come loose and the motors are OK.

If the connectors look good, check the wires where they go into each connector. If they are crimped, check the crimps, as they may need to be re-seated. Check the mating of the connectors too...mating elements (usually the female) may have shifted.

Worse case, use a multi-meter to determine whether all the motor wires are getting current when the controls are activated for movement If some aren't then back trace.

If you can't find any problems, swap elements between a working chair and the non-working chairs to nail down what part of the system needs to be replaced.