Three Computers - Internet Modem And Router Setup

We have three laptops in our house. We are set up with internet that gives me a direct connect and up to eight wireless devices. My son and daughter-in-law use the wireless and I use the direct connect. M son added a router (which I have no idea what that does) and set up networking on his computer (which I also have no idea what that does). When I am the only one on the computer, it flys. When either of them get on thiers too, it slowes a little. When both of them get on, I might as well get off because it can take up to three and a half minutes to load a page.