To respond to your edit (Which I don't think I quite explained):
In that situation, A doesn't care how long it actually took to find those blocks, the scaling of the difficulty/work-factor handles that. A can rely on the fact that (Unless someone introduced or removed a significant chunk of hashing power, or broke the hashing algorithm) the current network difficulty represents the amount of work needed to represent approximately '10 minutes'. PoW below that difficulty isn't accepted, and above that would be considered proving more work then required (But is also, of course, harder to find. So it proves more, but should take longer then 10 minutes for the network to find).
Since Bitcoin calculates the difficulty to result in about 10 minutes between blocks (As I explained in my last comment), that's how long it should approximately take. It's possible B or C got lucky and found it faster then that, but statistically that is unlikely since the hash result is completely random. If significantly more hashing power is added to the network and the blocks are consistently found faster then the target time, then when the difficulty is recalculated later-on (After the 2016 blocks are done) it will go-up, making them again take about 10 minutes with the hew hashing power taken into account.
In that situation, A doesn't care how long it actually took to find those blocks, the scaling of the difficulty/work-factor handles that. A can rely on the fact that (Unless someone introduced or removed a significant chunk of hashing power, or broke the hashing algorithm) the current network difficulty represents the amount of work needed to represent approximately '10 minutes'. PoW below that difficulty isn't accepted, and above that would be considered proving more work then required (But is also, of course, harder to find. So it proves more, but should take longer then 10 minutes for the network to find).
Since Bitcoin calculates the difficulty to result in about 10 minutes between blocks (As I explained in my last comment), that's how long it should approximately take. It's possible B or C got lucky and found it faster then that, but statistically that is unlikely since the hash result is completely random. If significantly more hashing power is added to the network and the blocks are consistently found faster then the target time, then when the difficulty is recalculated later-on (After the 2016 blocks are done) it will go-up, making them again take about 10 minutes with the hew hashing power taken into account.