Hi Ballantine,
I think this is what you should do :
All wifi WAN on seperate subnet say 192.168.2.0 , 192.168.3.0 , 192.168.4.0.
Connect the wifi WANs to managed switch with vlan tag 200, 300 and 400.
Connect the pfsense box to trunk port of switch.
Create vlan tags 100,200,300 in pfsense box. Create seperate vlan interface for each tag with interface ip as 192.168.2.1, 192.168.3.1 and 192.168.4.1 and upstream gateway as 192.168.2.0, 192.168.3.0 and 192.168.4.0 respectively.
For configuring the switch you should look into product manual. In general, in L2 switch there is one port configured as trunk port where pfsense box is connected. The rest of the ports to WAN with different VLAN tags. When the packet pass through them it attaches a vlan-id tag to it.
good luck and have a nice day
Ashima