I grew up in the middle of of the three peaks. So have done them a lot.
Individually they are ok, even for kids. As a set of three, they are easily do-able, but it can take it out of you a bit.
I did them last year with some mates. It was a birthday thing, and so we had champagne and sandwiches at the top of each hill, hip flasks all the way round and a pint at the Hill Inn pub. I did it in jeans, trainers and a tshirt, and carried a thin waterproof. The professional walkers looked really annoyed with me.
There are a couple of fairly steep bits, and one bit that is always really boggy. My friend thought he could jump the bog and ended waist deep in it.
11 - 12 hours is a sensible time scale for some one thats fit but hasn't done much walking.
Make sure you have comfortable shoes that can take the journey, And stuff too keep dry/warm if the weather seems that way inclined.
There is a lack of mobile reception for a lot of the walk. There is a patch where the reception returns and everyones phones start beeping.
And, amazingly, on a good day you can see the sea from the top of one of the peaks.