If you need separeate networks which should not to comunicate each other or
communicate by router only, there is no sense to connect all of them to
switch in the same segment or vlan.
If they are in different class and you connect them to switch directly you have
many "martian sources" and big mess with broadcast etc.
The point is not how to connect diffrent networks to 1 router interface but how
to run captive portal on more than 1 interface.
Thought for the day:
Intuition (n): an uncanny sixth sense which tells people
that they are right, whether they are or not.