To achieve this, a new VLAN will need to be configured and it will need to be assigned to a separate firewall zone.
Let's say you want LAN1 and LAN3 ports to be in the 192.168.1.0/24 subnet, while the LAN2 port will be in the 192.168.2.0/24.
Let's start by creating a new VLAN. This can be done by navigating to Network → VLAN → Port Based. Press Add button and a new VLAN will get created. On VLAN1 (first row), change the LAN2 port from Untagged to Off. Then for freshly created VLAN3 (third row), change the LAN2 port to Untagged. Press Save & Apply.
Now, navigate to Network → Interfaces → General, and create a new interface with the name lan2 (can be set to something different). Select the Protocol as Static with the IP address of 192.168.2.1. Set up the DHCP server. In the Physical Settings, select the eth0.3 as the interface and check the Bridge interfaces checkbox. Press Save & Apply.
Finally, the Firewall can be configured. Navigate to Network → Firewall → General Settings. On the lan zone, set the Forward rule to Reject and press Save & Apply.
And this should be it, if connected to the LAN1 or LAN3 ports, you will receive the address from the 192.168.1.0/24 network, and if you connect to LAN2, you'll receive the address from 192.168.2.0/24 address space.
Let me know if there are some further questions!