Black ops 3 strict NAT with UpnP enabled for multiple xbox ones

You have two routers (the sbg6782-ac and the R7000), which is really bad for gaming due to double NAT-ing.   Your first order of business is arrange the setup so that Xbox traffic is NAT-ed only once.

I'm not that familiar with the sbg6782-ac and the online manual is not very complete.  Do you know if it can be put into bridge mode?  This will disable the router functions.

Another possibility is to use the Pass-Through feature.  This feature allows you to designate one device to receive un-NAT-ed traffic.  You can use this to designate your R7000.  This thread on Shaw's forums provides details.

A third possibility is to convert your R7000 into an AP.  The AP Mode setting is under Advanced options.  Follow the instructions on screen or read the manual.  Once you enable this, all devices connected to the R7000 will need to acquire new IP addresses.

After you have done one of the three options, above, and have confirmed that basic Internet access still works, you should have one router performing NAT.  Log into that router and enable UPnP.  Remove all port forwarding rules and disable DMZ, too.  Finally, reboot your Xboxes for safe measure and see if NAT is open.  

Unfortunately, this will not guarantee that Black Ops 3 will work with multiple Xboxes.  Some games simply do not make effective use of UPnP and, therefore, will not work well with multiple Xboxes on the same home network.  If this happens, you can try one more thing.  If you didn't opt to use the Pass-Through feature, go back and enable it (switch your R7000 back to router mode, if necessary).  Then connect one Xbox to the R7000 and one Xbox to the xbg6782-ac.  Enable UPnP on both routers.  Each router will have to contend with only one Xbox, so things should work.  If you have 3 Xboxes, well, then you have my sympathies.