MSPROJECT – spreading hours over days

I have a task that is 6 hours to be performed over a 3 day period. But the days are changed back by the system to 1 since there are 8 hours defined in a day. If I move the days to 3 then the hours are automatically changed to 24.

This function is allowed if I use days as units but not when I use hours as units.

How can I define the task as 6 hours and ending 3 days after the beginning date?