As a minimum you connect the first disk using an ethernet cable to the main router. The subsequent 2 disks can either be allowed to establish their backhaul to the first disk by wifi or in my case I have them ethernet connected as well. Either will work well.
You need to stop using WiFi from your original router, ideally disable it to avoid interference, although I've reconfigured my Whole Home WiFi to use my original SSID and WPA2 key to avoid the need for re-configuring all of my devices.
It's early days so far, but it seems to do what it says on the tin - I now have a strong signal throughout my home and roaming is seamless, always putting me on the strongest AP as I move around.