CyanogenMod 10.1 RC1 dropped earlier in the month. The RC1 release brought an initial support for roughly 40 devices with talk about how it would be "one of (if not the last) milestone releases before a 10.1.0 is pushed out.” With that in mind, CyanogenMod 10.1 RC2 ended up coming available about a week later and as of today, RC3 is available for download.
The RC3 support looks to be about the same as what we saw with RC1. Those looking to update, or to join in one the CyanogenMod fun for the first time can hit up the get.cm website for the latest downloads. Alternatively, those already using an earlier 10.1 RC release can use the CM update app. That brings the question as to whether you should worry about upgrading from RC2 to RC3. Or alternatively, from RC1 to RC3.
The easy answer would likely come down to whether you are currently experiencing any issue. If you are having issues with your current build an upgrade may be the answer to your needs, but if things are running smoothly, you may be better served by waiting for the final 10.1.0 release. That said, the CyanogenMod Google+ posting did mention this is a "small incremental change over RC2."
There isn't a full changelog available, however details coming from that same Google+ posting point towards the following items being addressed with this CM10.1 RC3 release.
"addressing a few tens of issues all of you reported through Jira (including some global problems like the preview issues after using camcorder, bad launcher icon alignment in tablets, disappearing widgets, a crasher in the T9 launcher, and a ton of translation updates)"
Perhaps more important though, this just means we are now one step closer to seeing that final release. We do have to remember that the Cyanogen team doesn't ever provide time estimates for upcoming releases. But with the way they seem to be running through these RC releases, one may be safe to think the final will be here sooner rather than later.