How Can I Limit the Number of Selections Made in a Checkbox Field

I have a need to limit the number of checkboxes that are chosen in a form.

EXAMPLE:
(A) Choose 1 category
(B) Choose up to 3 categories

If the user chooses (A), so they can only pick one category, from a drop down box.

What I need to do, is limit the number of selections if they choose (B) as I'm displaying a checkbox list of categories. If I have 72 categories, I want them to only be able to choose a max of 3 (three), not all 72.

Is this possible? What is the easiest way to implement this?

Hi, this solution doesn't work for me. I put this code in functions.php and changed input ID, and nothing. Any idea?
code i put:
<script type="text/javascript">
jQuery.noConflict();
jQuery(document).ready(function($) {

$.fn.limit = function(n) {
var self = this;
this.click(function(){ return (self.filter(":checked").length<=n); });
}
$("ul#input_22_70 li input:checkbox").limit(2);

});
</script>