Different domain for each blog in multisite

We have a WordPress multi-site with 3 blogs (sub-sites), like this:

• website.com/subsiteA/
• website.com/subsiteB/
• website.com/subsiteC/

We would like to map each blog to a different domain, like this:

• subsiteA.com/
• subsiteB.com/
• subsiteC.com/

Notice, the second set (a) uses unique domains, and (b) does not show the subdirectory the blog is actually stored in. They each look like a unique site from the URL, though they are still running within the same WordPress multisite. Does anyone have any ideas for how to accomplish this (or either one, a or b)? Thanks.