Yes, the EOS 6D DOES support more than three bracketed shots. According to the Canon specs on their own website, a custom function can be set for 2,3,5 or 7 exposures, just like the EOS 1D Mark III and some other EOS models.
This is copied from Canon USA web site - http://www.usa.canon.com/cusa/professional/products/professional_cameras/digital_slr_cameras/eos_6d?selectedName=Specifications
Exposure Compensation
(1) Manual : ±5 stops in 1/3- or 1/2-stop increments
(2) AEB : ±3 stops in 1/3- or 1/2-stop increments
- Manual exposure compensation and AEB can be combined.
Manual exposure compensation and AEB can be combined.
- The AEB shooting sequence will be standard exposure, underexposure and overexposure. With the self- timer, three continuous shots will be taken regardless of the current drive mode.
The AEB shooting sequence will be standard exposure, underexposure and overexposure. With the self- timer, three continuous shots will be taken regardless of the current drive mode.
- The number of bracketed shots: 2, 3, 5, or 7 settable with a Custom Function.
The number of bracketed shots: 2, 3, 5, or 7 settable with a Custom Function.