Connect COM wire to three controllers

I have three Gen 3 controllers (16, 12 and 12 zones) that I am moving into the same physical location and running valve wires from five manifolds around the yard. I do not have a pump controller, moisture sensor, etc. just a straightforward bunch of valves to control. I read about the Easy Irrigation Isolator but that only allows one Controller to run at a time. I may want to run zones attached to different controllers at the same time since I have plenty of pressure and volume.

Can I connect the COM wire between the controllers and share it with ALL the white wires from the valves? Then I wouldn’t have to worry having a manifold with 5 valves with 2 zones on Controller 1 and 3 zones on Controller 2.

Yes, sharing the comms between three controllers without an isolator is fine as long as you use OEM power supplies, and don’t have to share the master valve, pump, rain sensors, etc…
In such a large setup as yours, I’m amazed that you forgo the master valve. Sharing one with thee controllers shouldn’t be hard or expensive if you ever wish to add some additional safety margin to your setup.