var recNum = 3;
window.addEvent('domready', function(){
    $('start').addEvent('click', function(event) {   
        var req = new Request.HTML({   
            method: 'get',   
            url: 'http://www.lhc.net/ajax-features.php',   
            data: {num_data :recNum},   
            onRequest: function() { },   
            update: $('log')   
        }).send(); 
		 recNum = recNum + 3;
    }); 
});