Some third person Games do not run at 21 by 9. So, if any game happen to not support the (21 by 9) aspect ratio I use the Nvidia control panel to set a custom resolution
I set the resolution at 3500 by 1987.
yes this is still 16 by 9 but you at least get rid of the side black bars. Still, though you will have a very small tolerable black bars at the top and and at the bottom.
Note: I didn't come up with the idea of setting a 16 by 9 aspect ratio using the Nvidia control panel I found it in a forum and felt like I should share it. (could not find my source though sorry).