In the set up pages for each of your routers, there will be a page on which you change the IP address of the router.
The address as stated above for each of them can be anywhere in the 192.168.1.___ range.
On the assumption that the new one is going to be connected to your ISP I suggest setting that to 192.168.1.254 , so that all the computersdevices you currently have are targeting the same number, whether they are using DHCP or not. On this router set DHCP to on and set the allowable range as say
192.168.1.10 - 192.168.1.150
My personal preference would then be to set the addresses for the other two to
in both of these DHCP should be turned off.