You can insert some custom text/HTML in your cart by adding the following lines to your View Cart code on every page, just before the "// -->" line in the standard View Cart code you obtained from Seller Admin:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#tdPmnt").attr("innerHTML",
"Ordered 3 items? Be sure to order your 4th <b>free</b> item!");
}
We don't have a way to display that only to buyers who have at least 3 items in their cart, but that might be possible with some custom javascript. If you'd like some help with that, we can recommend the competent, E-junkie-experienced developers listed in our directory:
http://www.e-junkie.com/ej/developer-directory.htm