Heidy, maybe you can do it using 2 timers. The first one for the starting time and the second one for the 3 hrs period.
You can also use the first timer and a Delay step of 3hrs, then a Fire step.
Rds
To the best of my knowlegde every process can be enable or disable by using ProcessName.Enabled. You can assign this state variable true at the beginning of the three hour period (not a must but probabily using a timer element) and false at the and of the three hour period. I hope it works?