After playing with it a little more, I realized that if I edit the SQL syntax to say something other than "NULL" at the line "o.date_purchased", it works. For example, now I have it like this...

SELECT DISTINCT c.customers_email_address as customers_email_address, c.customers_lastname as customers_lastname, c.customers_firstname as customers_firstname FROM TABLE_CUSTOMERS c LEFT JOIN TABLE_ORDERS o ON c.customers_id=o.customers_id WHERE o.date_purchased > subdate(now(),INTERVAL 3 MONTH) GROUP BY c.customers_email_address order by c.customers_lastname, c.customers_firstname ASC

The above will work...so my conclusion now is it has something to do with date purchased being zero or null.....Getting so close.....!!