How many hours at a time does one person work?
If there are 3 people sharing a job, working a 3 week rotating rota. Someone always has to work mornings and someone has to work lates. You can never have 2 people off at the same time. They have to take turns in having one weekend every 3 weeks off.
How can it be achieved for a work week to be 5 days in 7 without having to work stretches of more than a 7 days?