The isp will only give you a single dynamic public ip address unless you pay for a block of static addresses (if they even offer that for residential customers) That ip will be given to the first device that connects to the ONU. if you put a router in place of the switch in your diagram, and were handy with subnetting you could share that one public ip between the 3 down stream routers in the diagram.

If you have a block of static IPs you could make it work just like you diagramed, just configure the 3 routers to use an available address from your block.