I have one function: "update_user_like" to sum all facebook likes from all user's posts.
How to run that function at 3am every day by wp cron job?
First of all you have to remember how WP cron works. The action will trigger when someone visits your WordPress site, if the scheduled time has passed. So if your site isn't very crowded at 3am, the cron job won't start exactly at 3am. (You should use real cron if you really need to run it exactly at 3am).
To schedule an event at 3am every day you can use this code: