If we are talking about an ecommerce site:
On the payment confirmed page that the customer sees when completing payment successfully, you could email the admin if the stock level has dropped below three.
If there is a 'shipped' status for orders, you could instead perform the same check and email at that point.
Another option would be to have a daily cron job run that calls a PHP script. That script could generate a report on any items that are low in stock that you could then email to the admin.
