Prevent submission if entered date is less than 72 hours

On the date/time box. I don't want them to be able to submit a booking if it is within 72 hours of current date.

So if it's Monday today.

If they select anything in date/time before 72 hours it errors 

'For online bookings within 72 hours please call us on (Telephone nos)

To meet your requirements, we first need to calculate the difference between the date/time the form is being filled and the date/time that is being booked.

You may find this guide helpful in creating the calculation to get the difference between the two dates. Mastering-Date-and-Time-Calculation

You will be needing an additional date field in your form that has a default date set to "Current".

I will create a Demo Form that is based on your form and the requirement mentioned in this thread. Please give me some time, I'll update you in this thread as soon as I have successfully created the form.