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Right. In order for Mage A to counter B's counter, he'd have to start another spell...and you can't cast 2 spells at the same time.

The case of 3 mages is more complex. Mage A starts the CoC; B wants to counter. Can C stop B's counter?

I don't see why not. B is himself casting a spell, just doing so in an unusual way, and would be subject to all the usual issues...provoking an AoO, being subject to disruption, and...being subject to countering/dispelling.

One can rationalize the timing as follows:

--A starts casting; his spell starts to take "shape."
--B starts to counter. His counter starts to take shape. As a counter, it's shape is a 'reversed' form of A's.
--C starts to counter. Now B's spell "shape" is distorted, and it no longer matches A's. A's spell is now unaffected.

Taken further, especially with high-level spells, you could say that there's a risk of a really, REALLY bad "wild surge" or magestorm...a concept sometimes used in FR novels when LARGE quantities of high-level magic get used very quickly, as in a battle of 4 archmages. Imagine we've got a DBF being cast, 2 more being cast as counters, and 2 more mages using Improved Counterspell and using 8th and 9th level slots (the second Imp Counter would have to be a 9th)...all at once! I visualize gravity wells, or Star Trek warp fields, bending and twisting and crashing together...ANYTHING can happen. It's a lot like the classic Bag of Holding Meets Portable Hole...spacetime can't twist like that, and BOOM!!! Maybe the net result is some HUGE!!! surge...roughly determine the center of where ALL these interactions take place, and that's the center point for a 1000' radius, 100d6 (half fire, half untyped) explosion...or maybe ALL the casters simply have their spells lock up and feed back right onto them...they all take 15d6 fire damage, no save, and are stunned for 2d6 rounds. The exact effect is less important than the notion that Really Bad Things sometimes happen in such a situation.