As always, it is worth pointing out that "3D" would be a terrible name for any camera that did not do, you know, *actual* 3D. It would set up that exact expectation in consumers' minds. 6D would seem to make more sense if you want to buy into this rumor.
Hard to believe though. It doesn't fit neatly into the product line, and another FF seems unlikely given that they can't make enough (or in the case of the 1Dx, any) of the current ones.
Edit: lonelywhitelights said it first.