For one, you use Auctiva so let me assume you use Auctiva checkout. You will not be able to combine eBay winnings. For the future, turn off Auctiva checkout and turn on eBay checkout. This will not affect present listings, only future ones.
http://pages.ebay.com/help/sell/combined_payments_ov.html
In this case, log into PayPal, request money tab and use the send invoice feature to get around it for now.

But re-reading your question, you were able to combine two items? Then you ARE using eBay checkout? In that case, just send another invoice and check all three items, adjust the S&H and click "preview invoice". However, if the customer has already paid, you will not be able to combine the third item unless you refund the first payment, through PayPal using the refund link under that transaction.

Now back to the question that most people on the eBay forums ask about this secret way of doing business online, which is where do you find these drop ship suppliers? How do you get a genuine list of drop ship suppliers who have the product you want to sell and you pay wholesale prices only when the customer first gives you the order?

You can go online and do a search for drop ship suppliers, but once you go through each result, how do you know you can trust any of these companies? You don’t want to collect money from your customers and then pay a drop shipper who keeps the money but never sends out the merchandise.