All the product loaded from a loop script just initiate any $variable= 0 and after every product loaded $variable incremented by 1 and check $variable %3 == 0 if success add the div
e.g.
$variable = 0;
{ //product loop start
//product load
$variable ++; // variable increment
if($variable%3 == 0){
// Add Div here
}
} //product loop end
