I have to update a field with a value which is returned by a join of 3 tables.
Example:
select
im.itemid
,im.sku as iSku
,gm.SKU as GSKU
Did not use your sql above but here is an example of updating a table based on a join statement.
UPDATE p
SET p.category = c.category
FROM products p
INNER JOIN prodductcatagories pg
ON p.productid = pg.productid
INNER JOIN categories c
ON pg.categoryid = c.cateogryid
WHERE c.categories LIKE 'whole%'