Not done the three peaks challenge, but a few friends and I are walking Hadrian's Wall in may. We're going from Wallsend to Carlisle, coast to coast; around 90 miles, it should take us about 10 days with a bit of sightseeing along the way.
Time's ticking along and were out walking 3-4 times a week at the moment. My bergen is too heavy lol, however we are going to have a support vehicle which we'll meet at the end of each days trek so if all the kit is too much to carry and enjoy the walk, we can stash all the sleeping gear etc in the vehicle.
Charity is the aim of the game here too, but we've yet to get all of that properly sorted.
How fit are you? 3 peaks in 24 hours is an awful lot if your not very fit, plus the risk of an injury - knees, ankles etc.
Having spent the last 5 years sat behind a computer for work, I found that to start with the training to get your body used to that level of exertion was a bit of a shock; having pulled muscles and tendons in both knees at various times - a 16 mile walk with a day pack takes it out of you when you're not used to it, plus all of those hills and the driving rain and wind

despite the pain you just gotta put one foot in front of the other and keep on going.
This week coming we are getting all of the planning out of the way - food, accommodation (hostels, camp sites), kit lists, emergency contingency planning (we're not in the middle of nowhere, but it has to be done). On paper there's a lot to sort out.