How to Add Content Between Posts

in priciple, add a counter variable to the loop; use if statements to show the banner ads; increment the counter before the endwhile of the loop.

schematic:

<?php if (have_posts()) :
$counter = 1;
   while (have_posts()) :
      the_post();
      the_content();
if($counter == 2) { ?>
html code for A ad banner
<?php } elseif( $counter == 4 ) { ?>
html code for B ad banner
<?php } elseif( $counter == 6 ) { ?>
html code for C ad banner
<?php }
$counter++;
   endwhile;
endif; ?>

repeat as many elseif blocks as you need

http://codex.wordpress.org/The_Loop_in_Action
http://www.htmlite.com/php015.php

use custom field feature.