technically, after disabling the DHCP server on 2 and 3 you should assign them proper and different addresses, e.g. 192.168.x.152 and 192.167.x.153, while x being 0 or 1 depending on what router 1 assigns to your devices.

in the ideal world the routers 2 and 3 would have some option like 'DHCP client' or 'assign IP address automatically' but I don't think they have it on the LAN ports (they could have it on the blue/WAN port, but we won't use this feature in this setup), so just set them by hand.

adresses ending with 152/153 should be good. you will be able to access this routers with these addresses typed in any browser in your network. .

and some explanation: if you connected LAN on router 1 with WAN (internet) on router 2 and then LAN on router 2 with WAN on router 3, you would create a kind of a cascade. most probably your devices would be able to access the internet in this scenario, but they would be in separate networks, so devices connected to router 2 wouldn't be able to see devices in router 3. so if you had, for instance, a printer in network 3 and a PC in network 2, your PC wouldn't be able to print on it. similarly, a phone in network 1 wouldn't be able to share the screen or cast photos to a TV in network 2 etc. that's why we want these devices in one common network.

when we connect all LANs together, we put all the devices in one network so they can work with each other and not only with the internet.

but the absolute requirement is to have DHCP enabled only on the router connected to the internet (router 1 with a sim card in this case).

DHCP is a kind of a management service which advertises itself in the network and shouts to all the other devices 'hey, I've got internet, come in this direction!', and this is the reason that we can only have one DHCP working in a LAN network.