Ciao Andrea!
Ho provato come mi hai detto, ma non va...

Ho fatto così:
Codice PHP:
<html>
    <head>
        <script type="text/javascript" language="javascript"> 
            var conta = 0
            function disp(id){
                  var box = document.getElementsByTagName("input")
            for(var i=0; i<box.length; i++){
                    if(box[i].type== 'checkbox' && box[i].checked){
            conta++
            }else{
            conta--
            }
            }
            if(conta>0){
                      document.getElementById(id).style.display =  'block'; 
                    }else{ 
                      document.getElementById(id).style.display = 'none'; 
                    } 
                } 
        </script>
    </head>
    
    <body>
        <div id="cbox01">
            <input type="checkbox" onclick="disp('dsp1');" />
            Laurea
        </div>
        <div id="cbox02">
            <input type="checkbox" onclick="disp('dsp1');" />
            Diploma
        </div>
        <div id="cbox03">
            <input type="checkbox" onclick="disp('dsp1');" />
            Master
        </div>
        
        <div id="dsp1" style="display:none">testo per box 1!</div>
    </body>
</html>