Priority 1

You must comply with those. Without it, you are cutting out a bunch of people from being able to access your site. By complying with Priority 1, you have a mostly accessible site.

Priority 2

You should comply with those. If you don’t, you are making the life of many of your visitors much more difficult. By complying with Priority 2, you make a site that is free of significant barriers.

Priority 3

You may comply with those. If you do, you will have a majority of people able to use your site. By complying with Priority 3, you are really increasing access to your site.