linux – Reaching computers on second computers lan though network router

Using Linux.

Main computer ethernet adaptor eth0, ip connected to router, ip router connected to cable modem.

Second computer ethernet adapter#1 eth0, ip connected to router, has access to internet. Second computers ethernet adapter#2 eth1, ip connected to a network hub.

Two raspberry pis connected via usb->ethernet adapter to network hub. ips & everything on 192.168.3.* reaches each other.

I want to reach rpi @ from first computer What do I need to do on computer 1 & 2? I’m fine with using ip’s vs nameserver.