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. .. )
That would be useful to know. If you're just wanting to play content without modifying the files, then there are plenty of DLNA options for iDevices that can then be AirPlayed to the ATV. Air Video HD is a good example - the server side is free, the app is relatively cheap, it lets you access your content remotely & without file type restraints, and it supports AirPlay.
I've found it's just easier for me to make everything compatible with iTunes and setup my media server with iTunes running to manage it all. Even with a large amount of files needing to be processed, there are batch apps that will convert files to .m4v. I also know not everyone wants to do that.