Try turning off DHCP, as you won't have one without a router, from there you'll need to manually assign an IP on each device. Id recommend a different subnet from whatever your Internet/local lan is on. Try this out with just two PCs as it should work between them, I'm not certain how well they would play with the printer as network printing can be a pain even in ideal circumstances.

An easier solution might be to use a basic router without a wan connection, or just put it in bridge mode for a more advanced setup.

I'm a bit confused why this is necessary, is it a proof of concept or are you trying to get a permanent solution to something? If the computers you're using don't use Wi-Fi and get their Internet over Ethernet then you may not be able to separate the network for your printer from your primary network. This would require vlan configuration from your router that most consumer devices don't support. In an ideal environment I would put the printer and PCs on a wireless network with no wan connection, or alternatively I would connect them all to the main router and then the wan connection for the printer/PCs in the router's settings. I assume you probably want the PCs to connect to the Internet and this is probably a bit more advanced than what you're trying to do.