Power Purge valve from Dan Holohan, Pumping Away

You have 3 shown in the blue piping in that schematic.
Or you could use a single purge ball valve next to that blue manifold, delete the 3 single valves and boiler drains.

Isolation valves at the pumps, always a good idea, would allow you to purge one zone at a time.

The version with two purge valves would allow you to connect a purge pump for cleaning or adding chemicals