Taco SR503 switching relay and common wire?
Use a separate transformer, across Rc and C on the Nest

What I'd do here is have the Rh and W terminals on the Nests connect to the zone R and W terminals on the Taco zone board, while the Rc and C terminals on the Nests are connected to a 24VAC transformer (a standard 40VA one should be ample to power 3 Nests) that is independent of the Taco board.  This way, the Nest will turn off its internal Rh/Rc bridge and should power itself from Rc and C at that point, while it switches Rh to W to turn on the heat zones.  It also avoids overloading the internal transformer on the Taco, and should eliminate the need for isolation relays from the system.