You don't need a plugin. Update to LR4.2 at least, for tethered support with a 5d mkiii.
Thanks. The update helped, however it's still very buggy. Only hitting about a 20% success rate of LR loading the capture image and lots of "busy" messages in the camera. I'm doing some maintenance on the laptop, clearing some memory, etc. and hopefully that will help.
Are you using a Mac by chance?