Not a good idea to drive yourself, unless you want to be knackered.
Did it last year, raising £1.1k for charity. 23 hours 40 minutes.
Timings were something like:
Friday night - fly up to Glasgow, stay overnight in a hotel
Sat morning - meet everyone (we had 10 or so of us doing it)
Sat afternoon - drive to Ben Nevis from Glasgow
515pm Saturday - start Ben Nevis in glorious weather. Still snow at summit though. Did the tourist trail from the youth hostel.
Just shy of 10pm - get off Ben Nevis
Waste an hour getting food in Fort William (don't do this, seriously)
Drive to Scafell Pike - this took a fair few hours (6 or so) with a stop at Gretna services. Roads are **** coming down through rural Cumbria - watch out for sheep!
Scafell Pike - start at 4am, finish by 745am. Did the trail from Wasdale Head. Weather was atrocious at the summit.
Drive to Snowdon (stopping in Chester services). Get to Snowdon for 115pm. Climb Snowdon - up / down Pyg trail. Really, really poor weather. Cold, wet, windy, bad visibility. Witnessed a helicopter rescue on my descent.
Advice I've got:
* Get a dedicated driver. Our driver was able to chill out between driving. It's not fun to drive/walk.
* Get a largeish vehicle for this. Although be aware - the roads around Wasdale are narrow.
* Get maps / compass and learn how to use them (and GPS is also good). Sure, the trails are well defined (with the possible exception of Scafell) but you'll thank that you've got these when visibility decreases.
* Get decent kit - jacket / fleece / trousers / shorts / under layers / boots / socks. Change of under layers / socks between each mountain is a good idea. Especially if the weather is ****.
* Get a camel bak for water / other gear (survival bag, maps, compass, snacks, fleece, jacket) you need to pack.
* Don't overdo it on caffeine / sugar when doing it. Last thing you want to do is have a sugar crash on a mountain.
* Break your boots in well before the event. And get some time training in - I did Pen Y Fan a few times and Snowdon twice in one day before I did the 3 peaks. If possible, walk up 2+ of the mountains - I wish I'd done Scafell before the event as this is the most difficult to navigate).
* Plan your driving routes - there's a few really good sites online. Work out food requirements (any veggies in the group? This cost me an hour!) and fuel (both food/water for walkers and petrol/diesel)
* Get a good head torch, torch and red LED (2 quid in tesco)/glowsticks to attach to your pack for when you're in the dark.
* Summer brings busiest times on the mountain (you'll see several groups doing it on the weekend you're doing it), but the longest daylight hours. Only the very start of Scafell was in 'proper' darkness when I did it. Also best chance of good weather. I'd recommend the 5pm -> 5pm plan we did over an earlier start.
* Don't be an idiot. The mountain rescue teams in these areas have enough to deal with as it is! Pack proper kit and know how to use it. If you're not feeling up to it, stop. If the weather is really bad (unlikely in summer but who knows?), don't attempt it.
* If possible, don't use a minibus. There's a fair few miles of driving involved and being at 62mph will cost a fair bit of time.