A lot of information on the challenge can be found here: http://www.thethreepeakschallenge.co.uk (amongst many others - it is a popular walk).
You'll used a lot of energy in the 24hours so make sure you plan your food carefully - our group had big Tupperwares full of pasta salad which seemed to do the trick.
Having a dedicated driver (who is not walking) is a really good idea so you can sleep in between walking.
Make sure you are correctly equipped for the walking, even in the Summer the weather can get bad quickly. Experience of hill walking, navigation skills, compass, maps are necessary.