Multiple public ips with 3 routers sharing one modem connection

Looking for a simple solution if possible.
Have one modem with static ips that has 4 port switch on back on it. From it, has 3 separate routers connected to back of the modem, each configured with a different public ip.

RDP will not work when trying to RDP from inside lan of first public ip/router to the 3rd one but from a different wan connection works fine.  I know might be nat loopback policy needed but the 3 routers are on different lans and the only common thing between everything is the modem and basically the same gateway since sharing the modem. I know nat loopback usually points back to a object/ip/server,etc on the lan side but the device I'm trying to connect to isn't under this lan, it is under other router inside its lan.
Flyboy88Asked:
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ajeabCommented:
check static route on router 1 and 3.
on router 1  -- has route of 3rd network to router3
on router 3 -- has route of 1st network to router1

right now it look like it try to go out from computer1-->router1-->modem-->internet
you want to do computer1-->router1-->modem-->router3--->computer2
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