Colin McRae Rally 2.0

Use the same method of custom resolution as for 16:9.
The wider you want the picture to be the smaller you have to make the horizontal pixel count.
After some research the values I came up with are roughly:
16:10 - 1220:1080
21:9 - 836:1080 (the image will be stretched horizontally 3 to 1 - 836 to 2520)
3:1 - 617:1080 (the image will be stretched horizontally 9 to 1 - 617 to 5760)

This radical stretching will probably not look good.
You can use downsampling to compensate for this, so the image on 1080p monitor will be squashed vertically.
21:9 - 1672:2160
3:1 - 1234:2160