Nighthawk AX12 AX6000 max LAN speed

Something isn't reading right with your setup if you're reading 3gig up. As the CM2000 and xfinity don't support those speeds on the upload. 

to start off with, don't trust the built in speedtest at multigig speeds. It rarely will max them out. I'd recommend using xfinity's speedtest as that's been the most accurate for me on xfinity. 

The other thing is you won't see multigig speeds over the wired because it only has the 1 multigig port and you're using that for the wan port. What I usually tell people to do is run 2x wired speedtests at the same time. (if you have 2 wired computers). Or even just run 1 wired and wireless test. And again, test on xfinity's site.