I would have a look at the openwrt project’s database of supported devices. You can filter for devices with 3 nics (though not sure it supports filtering for “3 or more”).

https://openwrt.org/toh/views/toh_available_16128

use VMs. qemu/kvm. the Tor-based Whonix OS takes the approach of one VM running a Tor proxy and another VM running your application software. the latter VM only has access to that proxy, and no other network interface. it’s effectively the same approach as i understand a slug to be, but with the hardware virtualized instead of physical (or course you don’t have to use Tor — you can define whatever interface you want: a VPN, a firewall, etc).

I am using these:

https://www.seeedstudio.com/Rapberry-Pi-CM4-Dual-GbE-Carrier...

... to make firewalls and bridges with rpi cm4 ...