You have to use policy routing at both Router 1 and Router 2.
at Router 1, packets coming from 192.168.3.0/24 (or from ether4) must be handled by a dedicated routing table whose default gateway is Router 2's address in 192.168.10.0/24.
at Router 2, packets from 192.168.3.0/24 must be handled by a dedicated routing table whose default gateway is Router 3's address in 192.168.20.0/24.
You can use /ip firewall mangle or /ip route rule to assign routing-mark values.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.