Canon is just trying to keep you on your toes.

There are differences between all three.
Sometimes you have to look close to find them.

the SD800 has a wide angle lens (28mm focal length)
the SD900 does not have image stabilization.
the SD900 has 10 megapixels.

Here is a link to a comparison chart:

Examine every line in the chart to find all the differences.

As far as image quality.....all the same....very good.