A central heating plumber was installing a new gas boiler. When he commented on the pipework - I explained that my heating system was controlled by two different rooms' thermostats plus the hot cylinder one - but only used two three-port electromechanical valves. The effect was achieved by daisy-chaining the valves' "call" signals.
He then told me it couldn't work as three-port valves can only open one of the A or B positions at a time. He had never heard of Honeywell three-port vales that can open A or B, or also open both simultaneously.
The control box with lots of pretty indicator lights contains a schematic diagram - and an explanation of how it works. It still seems likely that some future occupant will hire a plumber who installs a replacement of a cheaper simple A-B valve.