In proper routing a route is literally just a destination network range and the gateway used to reach that network range. So you can't have a route without a gateway.
Dumping traffic onto an interface is known as a connected network and is using ARP, not routing, to handle traffic. So if you want pfSense to send traffic bound for 172.20.0.0/16 to interface 3 without a gateway then you need to give pfSense an IP in this range on interface 3 and make sure there are no layer 3 devices between pfSense and the client machines on this network.
I suppose you could put a proxy ARP device of some kind between pfSense and the 172.20.0.0/16 network and then create another /16 subent between pfSense and the proxy ARP device, but that would just make things complicated.