I need to allow people to book 3 hour appointments at any

I need to allow people to book 3 hour appointments at any time of the day.

For example, I have the day setup to start at 9am end at 6pm, and the appointment length set at 3hrs. Because of this, the app only allows people to book at 9am, 12pm, and 3pm.

I need to allow customers to book at any hour for their 3 hour appt, ie. customers should be able to book at 10am, 1pm, etc.