I have never head this idea described in text before. However, I have made firewalls this way for decades.
They were typically for stuff that ran in a datacenter so it would be a 1U server with three NICs.
I would really like to make such devices for home or office use. What would be a good device to use for this? Unfortunately, RaspberryPIs do not come with 2 or 3 NICs. Any recommended alternatives?