@Orangemintcream I would struggle to get my fat arse up one peak, never mind 3 in a day! A decent level of fitness is subjective. And you need to be able to hike at pace to ensure you can complete in 24 hours.
I read a report about a gym that had used this as a challenge for new members who had joined the gym with very low level of fitness (or very fat arses!) A bit like couch to 5k. Some of the group made the 3 peaks some made 2. And the ones who achieved the 2 peaks were very proud of their achievement. I can't remember the length of time they had been doing the gym sessions before attempting the Three Peaks
Most work groups who I know who have attempted this challenge have done some training beforehand