Need Help setting up an OPT interface and IPsec to 3 remote sites

[squatting_slav]

Hello all!
So I'm basically trying to set up the network from the diagram attached. I already have my WAN and LAN set up. I'm trying to set up an OPT interface on my pfSense box that is going to connect to an Ubiquiti ER-X box via some internal transit network. The ER-X will be connecting to 3 remote sites with IPsec tunnels. My goal is to be able to have clients from my internal LAN access all 3 of the remote sites and vice versa. Is this even possible? How should I set that OPT Interface? What kind of rules would I need? I appreciate all the help I can get on this. Thanks! :)