Our teachers can teach only three different week days in three weeks timetable

Let's take a case, when you have the timetable for more weeks and there are teachers, who teach different lessons in each week (usually some external teachers, or contractors). Then you were requested to distribute their lessons into minimum different weekdays.
For example, if you put their lessons onto Mon, Wed, Fri in the first week and then Tue Thu in the second/third week – they must come to your school every day in a week. So, in case, these teachers have other activities - they have each day blocked by you. So, it is more friendly to put their lessons on Mon, Wed, Fri first week and then again Mon, Wed second week. This way he will always have Thu and Tue free.
You can do so with this relation:

Note
With this relation you can save space for teachers, if they teach different lessons in more weeks timetable and you want to minimize usage of the different days in the week.