The setting lies in the source tab of the channel setup. Assuming your channel source connector is Database reader or a file reader.
Set the Polling type as Time
instead of interval. That will ask you to specify a particular day time eg 11:15 AM and everyday at that time the data will be polled.
If you are talking about 3 hours gap then set the polling type as interval
and put the value of 108000 ms(1000*6*6*3)
in polling frequency.