I'm surprised at you Graham. That's not up to your usual knowlegeable standard
The categorisation has TWO letters, not just one. The length of the race is denoted by the second letter - L, M, or S so the length of the Three Peaks should not be an issue when debating its A, B or C status. We have always been told that the A/B/C category is decided by feet of climbing and how much of the race is on true fell and how much on road/track.
Yes, the 3 Peaks is a hard race but based on what I understand of the FRA's categorisation system it's definitely a BL.
When you think about how much artificial surface you are running on I even think it borders a CL!!
The reason it has been given an A status is more to do with it's long-standing fame in the fellrunning world as a tough challenge. Should this justify it's A status? I don't think so - not if the categorisation is to be considered logical and not just subjective.