In general, if you have a Fri and Sat set as the weekend and a 3 night min LOS, any booking starting on either Fri or Sat will be required to stay 3 nights.

If you need to include a Thursday into your weekends then you will need to create a season for each Thursday of the year and put a 3 night LOS on that 1 day season. Thus, any booking starting on Thursday will be required to stay for 3 nights.