I repeat this message now and then here -- up to ISO6400 small pixels performs well. Only at ISO12800 and above current tech requires large pixels to perform well. So if you don't feel the need to shoot at ISO12800 - ISO51200 you should not need to be worried about high pixel count. Canon also have the sRAW and mRAW formats so you can get reduced size RAW files when you don't want the bulk of the resolution.
My guess though is that 5D3 will be a slow/crippled 1DX, and compared to D800 have less resolution (20-22 MP), less advanced AF, higher fps (6-7), slightly better ISO performance but still no setting above ISO25600 to avoid competing with the 1DX.