[deltaoneseven]

No exact answers exist but probabilities are very real.

3 people being simultaneously not fit for the job is quite unlikely.

You have a choice here. Either 1 person is unfit for the job or 3 people. 1 person being unfit though not for sure is Far more likely.

By one person I mean the manager is not fit. But in these cases usually the manager side steps the numbers and thinks he's doing the right thing.