Networks

"Connect 3 networks to a single printer, two companies in the same office"

I'm guessing that at least one of the networks is not physically connected to the other(s), because there are two companies. You haven't given much detail, and you don't indicate if you want to keep the networks isolated from each other, or whether it's acceptable to join them.

Easiest solution - flat topology - a big "party line" with everyone hooked together. Then use access rights to control who can connect to what. There are MANY reasons why this may be a bad solution, not the least of which are security and confidentiality.

Next easiest - separated addresses on a common cabling system. If each company has its own server, add another network IP on one to talk to the printer, then have the users use the printer share from the server, rather than connecting to it directly.

More secure/safer - establish a "bridge" for the printer from each network, and users are on separate cabling systems. A little more complicated, a lot more secure.

One thing you won't be able to tolerate is address overlap - if each company has its own address space, there should be no overlap.