I wonder what would happened to Canon user who switch to Nikon D800 if...

I think you will not regard switching to D800 even 5DX (something like) is launched (which in fact it is highly not probable because FF camera production line is narrow IMO), because

By the price of 5D3, how much would you think the new 5DX will be higher than D800?

From the current comparison of high ISO of 5D3 and D800, it shows that 5D3's ISO performance cannot match with D800's. How better will the new 5DX be in term of ISO performance in regard to the current ISO performance of 5D3?

The 5DX will price further higher than D800 and the ISO would be worse than D800. So you will not regard switching to D800 now IMO. Also, I don't think 5DX will be launched within 2~3 years.