No one has successfully jailbroken an Apple TV 3. Only Apple TV 2 can be jailbroken (using the free Seasonpass software from Firecore).
There is, however, a way to use "PlexConnect" to access a Plex server on your computer by hijacking the Trailers app on your Apple TV 3. I have never done this, but understand it allows you to access content in your Plex server (so for example if you have video files on your mac that are not in iTunes, this should allow you to play them without transcoding/remuxing to iTunes).
But that's more like a back-door trick, not a jailbreak.
As an owner of a jailbroken Apple TV 2, I can tell you that you are not missing out on much. The only jailbroken features I used were the ability to use XBMC to run the "freecable" and "Hulu" plugins, and even those don't seem to work any more. I'll probably just end up selling my ATV2 when Apple releases the new Apple TV (hopefully this year. .. )