Quick code snippet. Assume you have:

<p>Best Cheese? Please select the three:</p>
<div>
   <label><input type="checkbox" name="Cheddar" value="1" class="limitThree">Cheddar</label>
   <label><input type="checkbox" name="Cheshire" value="1" class="limitThree">Cheshire</label>
   <label><input type="checkbox" name="Creamy Lancashire" value="1" class="limitThree">Creamy Lancashire</label>
   <label><input type="checkbox" name="Cheshire" value="1" class="limitThree">Double Gloucester</label>
   <label><input type="checkbox" name="Red Leicester" value="1" class="limitThree">Red Leicester</label>
   <label><input type="checkbox" name="Stilton " value="1" class="limitThree">Stilton</label>
   <label><input type="checkbox" name="Wensleydale" value="1" class="limitThree">Wensleydale</label>
</div>

Then you could use the following jQuery:

$('.limitThree').on('change', function(){
	var noChecked = 0;
	$.each($('.limitThree'), function(){
		if($(this).is(':checked')){
			noChecked++;	
		}
	});
	if(noChecked >= 3){
		$.each($('.limitThree'), function(){
			if($(this).not(':checked').length == 1){
				$(this).attr('disabled','disabled');	
			}
		});
	}else{
		$('.limitThree').removeAttr('disabled');	
	};
});

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