WordPress 3.3 is one step further to being a reality. The developers hope this is the last step as well, now that the third release candidate has been made available.
If everything goes well at this late stage, WordPress 3.3 should be released to everyone within days, meaning that it should be powering all of your favorite blogs, at least the ones that care about their security, by Christmas.
If you develop for WordPress, making themes or plugins, it would be a 'really' good idea to test them with the latest version before it rolls out to the tens of millions of blogs that use the open source software.
WordPress 3.3 Release Candidate 3 should be the last one and should be the build that gets released as the stable version. However, there may still be bugs and, if you've waited this long, you can go on just a little bit longer and wait for the final release, unless you have real reasons to want to test the RC.