Yep, speaking only from a networking point of view (I'm new ot IP cams):
The IPs of the 2 cams plugged directly into the NVR would be on a different subnet and therefore not accessible ('touched') by any external device.
You can see them because you can 'touch' the NVR as it's plugged into the router; and the NVR can touch the cams.
The other cam can't be accessed externally as it's behind a router (NAT) and doesn't yet have portforwarding instructions to pass on requests to that IP.
The only solution I'd see here (someone might help) is if you plug each cam into the router directly; then set the stream to run on different ports for each cam (8081, 8082, 8083) then port forward each respective port to each respective local IP. ie. 192.168.0.81 to 8081.