MySQL Performance: MySQL => Bad Performance or Bad Query ?

I have three tables like this:

receipts

+------------+-------+------+------------+-----------+
| customerId | recID | plan | recDate    | processed |
+------------+-------+------+------------+-----------+
|       9742 |   900 | 1    | 1178726955 | 0         |
|       2188 |   899 | 1    | 1178670358 | 0         |
|       2657 |   898 | 1    | 1178639090 | 0         |
|       9699 |   897 | 1    | 1178627303 | 0         |
|       1247 |   896 | 1    | 1178570875 | 0         |
+------------+-------+------+------------+-----------+

customer

+-------------------------------+----------+-----------+
| Email                         | LastName | FirstName |
+-------------------------------+----------+-----------+
| user1@domain.com              | lname    | fname     |
| user2@domain.com              | lname    | fname     |
| user3@domain.com              | lname    | fname     |
| user4@domain.com              | lname    | fname     |
| user5@domain.com              | lname    | fname     |
+-------------------------------+----------+-----------+

user

+----------+--------------+-------------+
| Approved | UserUpgraded | UpgradeDate |
+----------+--------------+-------------+
| 1        | 1            | 1176134955  |
| 1        | 1            | 1176078358  |
| 1        | 1            | 1176047090  |
| 1        | 1            | 1176035303  |
| 1        | 1            | 1175978875  |
+----------+--------------+-------------+

each table is actually much bigger than that, over 10,000 entries ... now i need to get all three table's info at once ... since there is no index associated with any of those (poorly designed ... not by me) i am forced to use the WHERE clause.

now this is where i get messed up ... this query:

SELECT  
	DISTINCT receipts.customerId, 
	receipts.recID, 
	receipts.plan, 
	receipts.recDate, 
	receipts.processed, 
	customer.Email, 
	customer.LastName, 
	customer.FirstName
FROM 
	`receipts`, 
	`customer`
WHERE 
	receipts.customerId = customer.CustomerId
GROUP BY 
	receipts.customerId
ORDER BY 
	receipts.recID DESC
LIMIT 
	5;

produces the following result:

+------------+-------+------+------------+-----------+-------------------------------+----------+-----------+
| customerId | recID | plan | recDate    | processed | Email                         | LastName | FirstName |
+------------+-------+------+------------+-----------+-------------------------------+----------+-----------+
|       9742 |   900 | 1    | 1178726955 | 0         | user1@domain.com              | lname    | fname     |
|       2188 |   899 | 1    | 1178670358 | 0         | user2@domain.com              | lname    | fname     |
|       2657 |   898 | 1    | 1178639090 | 0         | user3@domain.com              | lname    | fname     |
|       9699 |   897 | 1    | 1178627303 | 0         | user4@domain.com              | lname    | fname     |
|       1247 |   896 | 1    | 1178570875 | 0         | user5@domain.com              | lname    | fname     |
+------------+-------+------+------------+-----------+-------------------------------+----------+-----------+
5 rows in set (0.01 sec)

which is fine ... but as soon as i add my third and last table into play .. the query takes about 2 mins to process and uses 50+% CPU ... this is the query i try to run:

SELECT 
	DISTINCT receipts.customerId,
	receipts.recID,
	receipts.plan,
	receipts.recDate,
	receipts.processed, 
	customer.Email,
	customer.LastName,
	customer.FirstName, 
	user.Approved,
	user.UserUpgraded,
	user.UpgradeDate 
FROM 
	`receipts`,
	`customer`,
	`user` 
WHERE 
	receipts.customerId=customer.CustomerId AND 
	customer.Email=user.UserEmail 
GROUP BY 
	receipts.customerId 
ORDER BY 
	receipts.recID DESC;
LIMIT 
	5;

am i doing something wrong ? any help would be appreciated

p.s. the reason im doing distinct and group by customer id from the receipts table is because there are multiple receipts for each customer ... and i want to get the latest one!

p.s.s. the user table also has an email column, thats how the user and customer tables are linked ...