In the past I believe the AF was either controlled by the main CPU (which generally hadn't been the digics, but an additional chip AFAIK) or an additional AF only CPU.
On the 1DX they will use a Digic iv as the AF chip.
Most of the rumors have mentioned single digic V for image throughput and some have also mentioned digic iv for AF. Who knows if any of this stuff is for real, the 22MP or 7fps either.
Anyway I don't think 3 digics, if it came to that, shoud be a shock in the general sense (coming from Canon, yes, perhaps). How much do you think a digic iv costs? they use them in the lowest of lowest priced Canon P&S cameras. And, I forget, but I thought they added a separate CPU for AF even in the 7D, although not a digic chip.
Why is it so insane to think (other than because of the sort of company Canon has become)? Other companies do it. If you think it is all insane, in principle (in reality, perhaps you are correct), you've had too much of the Canon kool-aid.
Definitely not drinking the Canon kool-aid, far from it.
Let's drive on that idea of 3 processors. Could all that fit in a 5D body? I'm thinking no, but.....it's possible. Looking at the circuit board images of the 1Dx, I have a hard time thinking that hardware is going to fit in a 5D body. Gotta remember, that 1Dx has the advantage of the room offered by the built-in grip.
In the 7D it uses the dual Digic's for the AF as well....check the specs and the circuit board layout...nothing indicates another AF processor in addition to the Digics.
I'll have to check but I could swear they said they added a little chip for 7D AF.
Well the 7D proves they can fit two digics and an additional master cpu and maybe a small additional AF CPU.
However, the digic chips do seem to take up more surface area than any of the other chips. Is there room to fit in three digic chips? Not sure. I'd have to look at the boards and think. It's possible you could have a point there if there is nothing else they can shrink or move around.
OK, let me see, well a quick bit of google quickly turns up lots of references to a dedicated AF chip being used in the 7D. So I think it has, but I think this chip must be noticeably smaller than a digic sin emy vague recollection was a circuit board with two big digics and then a bunch of much small chips, if I recall correctly (and I might not be).
I managed to find a very, very tiny pic of the 7D mainboard, it's so small that it is hard to say, it looks like they might just be able to cram in another digic, but it depends. Image it too small to really tell and perhaps the Digic 5 is much larger, etc.