Am doing my own challenge this year (John O'Groats to Lands End on my bike starting in about three weeks time), so forgive me if we agree to each sponsor our own causes.
I did do the 3 Peaks in 24 hours 1999. It's a tough challenge, but do-able if you're moderately fit. Tips are:
Good boots worn in and don't look to save money on socks - get good ones. You can get lots of other things wrong as long as you don't get blisters. At this time of year lightweight fabric / GoreTex boots would be ideal.
Traveling between the hills you need to stop regularly and stretch a lot to avoid stiffening up and cramp. Take bananas - I find they prevent cramp.
Take Sudocrem - you will get nappy rash.
You don't need a map - all of the hills (with the possible exception of Scafell pike, which you will probably do in the light) is like a motorway, you can't get lost and loads of others from all over the country will be doing the same thing as you, let them carry all the maps, just follow them.
Don't carry a rucksack. You will be sprinting up and down each hill, so wear a lightweight waterproof jacket and use the pockets - a pack of sandwiches, half a dozen snickers bars and a Camel hydration pack is all you need.
Watch out for midges in Scotland. You won't be bothered by them on Ben Nevis, but at the bottom and during the approach they will eat you. Nothing works that well other than covering yourself up - I use a Buff (like a lightweight snood).
Good luck.
