We would suggest composing a separate page on your site for this particular promotion, showing Cart buttons for only those three items, then your promotional email would contain a link directly to the URL of that page. You can even leave this page unlinked from the rest of your site, so the emailed link would be the only way to reach it. Once the buyer adds their chosen item(s) to the cart, they can click the PayPal checkout button in the cart to proceed to the PayPal checkout page for their item(s).