In the rule settings, set the Shift start from the current moment checkbox.
Specify a period of 3 days.
This will postpone the start of the rule for three days from the current moment.
If rounding is not used, the time will be locked smoothly throughout the day.
To block the time in 1 hour steps, select Round up to the beginning of the hour.
To lock the full day at once, select Round up to the beginning of the day.
In the latter case, you will need to increase the shift by 1 day.
In our case, you need to set 4 days.
If you need the manager to be able to make reservations without these restrictions, go to Parameters.
On the Reservations settings tab set the Disable dynamic working time range of rules for manager's reservations checkbox.
Check the result on the widget: