If I remember correctly, cmdline3.txt was supposed to be for Pi 3 units.
I've found the overscan settings, working with them now... Done. Took a bunch of guesswork and reboots.
The Pi 3 I'm working on for now has an ARM7 architecture, so there's that.
It's accepted my boot code for user name, but now it wants a password I never gave it. Holding CTRL+C while it was Loading extensions... dropped me into root login, though. I'd be curious about what would have happened had I had a root password, though.