You would need some hand coding to get it working. Here is some sample code that should work. Just add this before the server validation code:
<?php
$checkedBoxes = 0;
if (isset($_POST["study_print"])) $checkedBoxes++;
if (isset($_POST["study_digital"])) $checkedBoxes++;
if (isset($_POST["study_combined"])) $checkedBoxes++;
?>
Then you can validate the value of <?php echo($checkedBoxes); ?>
Using number validation to validate that it is greater than zero.