All 3 main paths up Scar Fell, Snowdon and Ben Nevis are well marked maintained and straight forward. I'd suggest anyone resonably 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 your likely going to spend 9 hours walking and 10 hours driving.
I'd say this is possible for pretty much any fit and healthy person and 1000's complete it each year.
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
- 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 your 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.
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.