I done the three peaks for MS back in 94 and Ben Nevis in 95 for the same cause, but the three peaks was not done in 24 hrs due to having two coach loads of folks at the time, Snowdon is the easiest peak up the pyg track as said done that in 1hr and 8 minutes, scaffell Pike in 1 and a half hours and Ben nevis in just over two hours. That time does'nt include the descent time and we started off at Ben Nevis.
Tips take plenty of deep heat or something similar as it does help in between climbs

Also on the descent from the Ben you can avoid the zig zag path on descent and just run down the scree which should take you out below the last zig zag and back onto the track before you hit the red burn (assuming you are taking the Pony Track). As there is no other quicker route up!!
As for Scaffell we went in from seathwaite 4 miles took the same route up and descended the same.
Snowdon the quickest way is up the pyg track and down is by the Llanberis path practically follows the railway line.
Good luck
