The 5D MK III is only rumored, and, no one will know about it right up until a week or two before its announced. Actually, there may be some accurate leaks, but there are so many who make up their own leaks or wish list that its impossible to determine if there is anything accurate out there.
We all believe it will likely be announced in a year or less. I would suggest that you trade in a old broken film SLR using the Canon Loyalty Program for a Refurbished 7D. I did, and mine was basically new, but cost a lot less.
When time comes to upgrade, you will be able to sell it at minimal loss. Just make sure that video accessories you buy are going to be ones that can be used on a future camera, high quality mic's, lighting, tripod and head, lenses, ND filters, external monitors, audio recorders, etc. Thats where the real money is spent if you are into serious video.