three old local networks via ISA 10MBits NICs
Are you sure you mean networks?
Or do you mean hosts (actual computers)?
Anyway, that would make the difference,
between either getting a switch or a router that is...
Then hookup your networks/hosts + sme to switch/router.
Offcourse the routing would take some configuration, with subnet's and so on... I'm guessing you meant hosts though...?
As said before, you may experience problems with your sme if you use an isa nic in there aswell (local network interface),
best check the hardware compatibility list, it's...err...easy to find with search

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