John,

For 1 and 2:

select customerID from purchases t1 where
itemID = 1 and exists (select customerID from purchases t2
where t2.itemID = 2 and t1.customerID = t2.customerID)


For 1, 2, and 3:


select customerID from purchases t1 where
itemID = 1 and exists (select customerID from purchases t2
where t2.itemID = 2 and t1.customerID = t2.customerID)
and exists (select customerID from purchases t3
where t3.itemID = 3 and t1.customerID = t3.customerID)



Jeff