I need to allow unrestricted access to an internal LAN based device from 3 specific external subnets.

How do I open up all ports to this internal device from these 3 external subnets?

I have set up the three subnets as ip objects and then added the all to a opject group.

What do I do next? What is the easiest way to do this?

Any help would be apreciated.

Use

NAT -> DMZ Host and then set up some firewall rules?

(AIUI, objects and groups don't do anything in themselves, they're just there to improve readability / maintainability)