All 3 main paths up Scafell Pike, Snowdon and Ben Nevis are well marked maintained and straight forward. I'd suggest anyone reasonably fit should be able to do anyone of these (up and down) in 3-4 hours (my fastest time up and down Snowdon is 2 hours). The hardest bit of the 3 peaks is the driving. Google thinks it's about a 10 hour drive so you're likely going to spend 9 hours walking and 10 hours driving.. Can you walk for 9 hours? Can you endure sitting in a car for 10?! :)
I'd say this is possible for pretty much any fit and healthy person and 1000's complete it each year. But an increasingly large number are getting themselves into difficulty, mostly due to poor planning. Have a look though the various mountain rescue teams blogs and you'll see a lot of 3 peakers getting lost, etc. and having to be rescued by mountain rescue (Llanberis MRT blog).
Many locals now dislike 3 peak teams due to some bad practices, always follow leave no trace ethics.
Here's a brief checklist of what you should be prepared for/be aware of:
- I'd invest at least as much effort in the organisation side of the
driving as I would in training for the walking.
- Be aware of daylight! Lots and lots of people get caught out by the sun going down.
- Know your route, have a map and compass with you.
- Make sure at least one member of your party is comfortable
navigating in the hills potentially in the dark, especially this
time of year (you've missed the longest days now).
- Injuries, carry a good first aid kit and be prepared to turn around
if you need to.
- Parking, sounds silly but this time of year all 3 mountains are
incredibly crowded. I know it's virtually impossible to park in
Pen-Y-Pass (the closest car park to snowdon) pretty much any time of
year. Ben Nevis is very remote and has very limited parking. So plan how you're going to get to the foot of the mountains. You
may need to use public transport unless someone can drop you off and
park further away. If you park badly expect a ticket (and possibly a shouting at from a disgruntled local)
- Plan toilet breaks. Don't defecate into the bushes, etc.
For those interested a much more environmentally friendly version is the Yorkshire 3 peaks, no driving between the mountains. Just because the hills aren't as big doesn't make it easier BTW, I did this in training for the welsh 15 peaks (more on my blog :) ) and it was pretty gruelling taking about 10 hours of solid walking.