I am not sure how to run a cron job at 3 specific hours every day. I want to run it at 1pm, 2 pm and 3pm.
Is it possible, using a single expression?
As lenik stated, it can be done in single expression.
0 13,14,15 * * * <your-script-to-run>
Check this geedkstuff link for more examples
While the given answers are correct, an unexperienced user might not know where to put this expression. You have to edit the crontab file, like:
crontab -e
There you add
0 13,14,15 * * * /home/user/command
to execute your command at 13:00, 14:00 and 15:00. Also note that user has to be substituted with the user account the command is executed in.