I need this on occasion. You must route the network IP/subnets back to each router on the downstream router. What are the IP/subnets assigned to each of these router interfaces? That would be the P1 and P2 on each router.
WAN <--> P1-R1-P2 <--> P1-R2-P2 <--> P1-R3-P2 <--> PC