you can do this for the last 3 hours...here are the steps:
1) create a new formula with syntax like this (substitute the {your ticket datetime field} for the ticket date time field that you have)
if {your ticket datetime field} >= dateadd('h', -3, currentdatetime)
and {your ticket datetime field} <= currentdatetime
then 1 else 0
2) place this formula in the Details section
3) right click on the formula, choose Insert > Summary and then for the summary location choose your group
now you will have a count of the tickets created in the last three hours.
cheers,
jamie