@sminded well if they are not under your control - connecting them together via a router, you control - normally not smiled upon by the people that control those networks. But sure you can connect any networks you have connection to together.. Routing stuff from one network to the other prob going to be problematic because devices most likely point to some other gateway provided by the people that manage that network.. So they wouldn't even send traffic to pfsense anyway to get to the other network.. Unless you manage devices on both of these network and could point them in the right direction to get to the other network, or used port forwarding and nat to allow them to talk to things on the other network. But since you don't want them talking to each other.. What is the point of connecting them together via pfsense?
If you want some device to talk to something on lan1 or 2 from 3 (your wan of pfsense) you would need either setup routes on these devices on lan 3, or use nat, and then you would have to also outbound nat it to either lan1 or 2, or why would those devices send the return traffic back to pfsense.