How to run cron job on a specific hour every day?

What do I need to write in crontab to execute a script at 3pm every day?

You are looking for something like this (via crontab -e):

0 15 * * * your.command.goes.here

15 is the hour and 0 is the minute that the script is run. Day of month, month, and day of week get wildcards so that the script gets run daily.

Here's a header that's good to put on top of your crontab for easy reference:

# +--------- Minute (0-59)                    | Output Dumper: >/dev/null 2>&1
# | +------- Hour (0-23)                      | Multiple Values Use Commas: 3,12,47
# | | +----- Day Of Month (1-31)              | Do every X intervals: */X  -> Example: */15 * * * *  Is every 15 minutes
# | | | +--- Month (1 -12)                    | Aliases: @reboot -> Run once at startup; @hourly -> 0 * * * *;
# | | | | +- Day Of Week (0-6) (Sunday = 0)   | @daily -> 0 0 * * *; @weekly -> 0 0 * * 0; @monthly ->0 0 1 * *;
# | | | | |                                   | @yearly -> 0 0 1 1 *;