See This previous forum post.
If you have tried the config.txt stuff (you have only mentioned commenting out the overscan lines and not actually changing the resolution, 7" Display is 800x480) and it hasn't updated after a reboot then see the answer below:
Just run the following simple command on Raspberry Pi 3 running Raspbian Jessie.
run terminal and type
sudo raspi-config Go to: >Advanced Option > Resolution > just set your
resolution compatible fit with your screen.
reboot If you didn't found the menu on configuration, please update
your raspberry pi software configuration tool (raspi-config).
That's all TQ.