I think you want to floor the time to the same value for each 3 hour range.
This works by
Floor( :date / (3 * 60 * 60) ) * (3 * 60 * 60)
assuming :date is the original column.
First the original date is divided so that 1 is a step of 3 hours (3 hours x 60 min x 60 s, JMP calculates in seconds), than floor is applied, than it is calculated back to the original range.
Georg