Running a cron job 3 times (1 pm, 2 pm and 3 pm for example)?

you may use this:

# m   h   dom mon dow   command
0 13,14,15 *   *   *     /home/user/command

your /home/user/command will be run at 13:00, 14:00 and 15:00

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.