Are you using the 24 hours calendar as the Project calendar in Project Information? Do you have resources assigned? Are those resources assigned the base calendars of day and night shift?

If you are assigning the day and night shift calendars to the resources, and then assigning the resources to the tasks, Project will only schedule the resources during their working time. So, if I have a 36 hour duration task and assign a day shift person - the task will span longer than 36 continuous hours - the resource only works 12 hours per day - so the 36 hours spans over 3 calendar days.

If you want to be able to have a task run continuously for 36 hours then you are either going to have to assign both Day Shift and Night Shift resources or forget the day and night shift divide and just use the 24 hours calendar to schedule everything.

Ah, sorry - I saw your second post about no resources after writing all this.

So going back.

If you have the 24 hours calendar set as the Project calendar in Project Information and you want to be able to enter "3" to stand for 3 - 12 hour shifts, your definition of day needs to be 12 hours, week of 84. Duration will show as 3 days.

If you want to enter 1.5 days to stand for 36 hours (3-12 hour shifts) then set your day = 24 hours, week = 168.

Changing those values AFTER entering tasks will not recalculate the start and finish date/times - it will just show the duration in the new unit.