I'm skeptical of an 18 mpix sensor for 5D3. Why would Canon put its new flagship 1Dx sensor in a lower class body right away? Doesn't make sense to me. I'm still thinking 36 Mpix. Canon will respond to Nikon in this regard.
The 1DX is in rather a different situation to the 1Ds mk3/5D mk2 though, the flagship body this time is being marketed more to a sports/jurno/wildlife crowd with both FPS and AF being big selling points aswell as the sensor
A 5D mk3 with 18 megapixels, and maybe marginal improvements in AF and FPS sold relatively cheaply wouldnt really be much of an alternative to the 1DX for those users. I could see it being very sucessful at the lower end of the market aswell since combined with a 24-105 it would provide a very versatile system without the need for large appatures.
I do have trouble believing that Canon will just give up on megapixels though given that they've released a number of lenses clearly geared to the studio/landscape market(and indeed to coping with the effects of difftraction) in recent years such as the new versions of the 14mm and the TSE's. The problem I'd say might be that trying to please former 1Ds users and FF newbies with a single body is simpley not possible. If Canon were to fudge it then they could end up with a camera too expensive to appeal to the lower end of the market and not high spec enough for the higher end.
The 1DX to me seemed like less an effort to merge the 1D userbase and more an effort to merge the sports, jurno,wildlife userbase onto one body(and get them all buying more expensive lenses). I could see Canon doing the same thing with the landscape/studio market aswell, ditch the dual grip and high end AF from the 1Ds that most of those users don't need and give them a 5D sized body with 1D build, a 100% viewfinder and high megapixel count that they do at a cheaper price, but still one significantly more than tradisional 5D levels.