First, let's see how much work you need: 6 days *3 people * 12.5 hours =225 working weeks.

Obviously, if four people share the work, each person can only work for 40 weeks, and you can only provide 160 working weeks, so there seems to be no way to solve the problem.

Unless you can only work 9 hours a day, your total workload is 6*3*9= 162 working weeks.

The arrangement is as follows:

A 1 2345-4 half a day in the morning =9*4.5=40.5 working hours.

B 2 3 4 5 6-4 Half a day in the afternoon =9*4.5=40.5 working hours.

C 1 3456-5 half a day in the morning =9*4.5=40.5 working hours.

D 1 2456-5 half-day in the afternoon =9*4.5=40.5 working hours.

So only three people go to work at the same time on Thursday and Friday.