I can have Dawnguard and Dragonborn loaded, even at the same time, but when I try to load Hearthfire with them or by itself, the Creation Kit crashes and shows this error:

The usual solution for this is to add certain things to the SkyrimEditor.ini file:

And that seems to work for most people, even in 2013, but not for me. I've even tried removing the Strings folder and validating the Creation Kit through Steam, and then validating it again with the Strings folder back in, each time editing the SkyrimEditor file, but it still crashes.