Just to give a little more detail to the answer above:
To combine the first two tables, go into design view for table1 and add any fields that only exist in table2. Then, create an update query to move the data from the second table into the first. Assuming there's only one extra field in the second table, called 'attribute', it'd look something like this:
I'm afraid I don't quite understand the third table - what's a cross sell item? As meratigoerr says, if one item number can have more than one cross sell item (ie the same item number appears more than once in the table) it's not a good idea to combine it with the first table. If each item can only have a single cross-sell item then you can follow the same procedure as given above.
UPDATE [table1] INNER JOIN [table2] ON [table1].[item_number] = [table2].[item_number]
SET [table2].[attribute] = [table1].[attribute];