How would I exclude certain categories within my query?

Thanks for any help with this.

My client has a cart for which I've made a custom homepage.

We use this query to display the 3 newest items on the homepage:

$query = 'SELECT * FROM `tbl_item` GROUP BY `item_id` ORDER BY `item_id` DESC LIMIT 3';

Now, my client has several categories which he does not want the items of to be displayed on the homepage.

In the database the column is "category_id" which is within the same table as in the query shown above.

How would I exclude several categories from being displayed via the query above? Thanks.

edit: I found this code will exclude 1 ID, but I need to exclude multiple ID's, not sure how.

$query = 'SELECT * FROM `tbl_item` WHERE NOT (category_id = "138") GROUP BY `item_id` ORDER BY `item_id` DESC LIMIT 3';

Ideally, I would like to exclude a whole parent ID, so I just have to put one ID in the query, but the parent_id is in another table, the "categories" table, not the items table.

I believe I have to do a subselect, but I don't know how.

where category not in (x,y,z)
would exclude those categories.

Thank you, that did it! :)