If you have 3 sites, and you have the ability to make a product link. Then show the picture of the "one" item. They show a price, but have them click the link to the actual location on one of the sites the item is listed. If the item is sold, then the inventory for the item will show zero, no matter where they came in from to where the item was located.
Recap: When you list the item, it gets located on one of the 3 websites. That item on that website has an inventory control so that when it it sold, it goes to zero. On the other 2 wesites, you post the pictures, the price, description and a link to where the actual item is website located.