INTERNET ----- VIC0 x         x VIC1 ----- x CN1
                    | OpenBSD |
                              x VIC2 ----- x FN2


pf.conf:

ext_if = "vic0"
int_if1 = "vic1"
int_if2 = "vic2"

...

nat on $ext_if inet from ($int_if1:network) to any -> ($ext_if)
nat on $ext_if inet from ($int_if2:network) to any -> ($ext_if)
nat on $int_if1 inet from ($int_if2:network) to any -> ($int_if1)
nat on $int_if2 inet from ($int_if1:network) to any -> ($int_if2)