Not quite in the exact way you describe, but in your Seller Admin > Edit Cart Discounts, you can configure order-quantity-based discounts, where an order for, say, any 2 items would automatically get x% or $y off, and any 3 items would get p% or $q off, etc. If you configure your products with "Allow buyers to edit quantity" disabled (so they can only order one of each at a time), this would be pretty close to the discount scheme you described.
Now, as to promoting the discounts, you could simply put some words/images to that effect on your site pages, and you can even add custom HTML to your cart screen which can be unique to each page in your site where the buyer is displaying the cart -- e.g., the cart viewable from Product A's page could display links to buy B and C, and the cart viewed from B's page could display links for A and C, etc. On each of your pages, you would add these lines to our standard View Cart code, just before the "// -->" line:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#tdPmnt").attr("innerHTML",
"Anything <b>HTML</b> you want to show");
}
(Note that "Anything <b>HTML</b> you want to show" can be different on every page, must be strung as one long line of HTML code, and must have any "double quotes" in your HTML changed to use 'single quotes' instead.)
