3 rooms, different locations; one ISP in one location

This is going to sound sort of stupid, but I suspect you already considered a wireless router and network interfaces. So what problems did you experience with the setup?

IF at all possible, it would honestly be best to run ethernet cable within the walls to the end points. I know that it takes a bit of work and you may not own the building, or know how / be willing to do the work. If you own the house, and don't know how which I suspect since you posted the question you did I would recommend...

#1. Purchase a good quality broadband router, again WiFi is still a good thing. Wireless AC is kind of a moot point at this point, but Wireless N dual band 600 / 750 / 900 models are plentiful, and support most modern equipment. Whatever router you buy, make sure it has Gigabit Ethernet ports. The Asus RT-N66u is an excellent N900 router but a bit pricey. The Buffalo High Power WZR-600DHP is an N600 router that is another very good piece of equipment, at a much more reasonable price point. The Netgear WNDR3700 is similarly priced, and featured, another good router. (I personally prefer WiFi routers with external antennae as they allow for better tuning).

#2. Hire a cabling contractor to pull all of your network cabling to where the router is. If you need more than 4 drops you will want to add a switch to the room where the router is, so it would go modem, router, switch, cables, devices.

#3. Any computers connecting to the network will need a Network Interface Controller (NIC). Unless you are running ancient computers, modern Desktop PCs will have PCIe slots available, and very likely the computers have built in network interfaces. Look on the back of the computer for the holes that look like a very fat telephone jack.

#4. Assuming your modem has an ethernet port on it, and NOT a USB port, connect your ethernet LAN port on your modem, to the ethernet WAN port on your router per the MFG direction sheet.

#5. Per the MFG direction sheet configure your router. Make sure you have DHCP and DNS working. Those will be critical for your connection. Also be sure your firewall is working, that will be critical to protecting your network!

#6. Cable router to switch if needed, or to wall plate to the ethernet being distributed through the house, and cable from wall plates to computers.

#7. Set client computers to DHCP client mode in the network settings. How to do this will vary depending on your operating system and version.

#8. Test, and enjoy...