checkbox.onclick = function(){
 //代码块
}

if(chckbox.checked){
 //代码块
}

< input type= "checkbox" name= "sport"/>篮球<br />
< input type= "checkbox" name= "sport"/>足球<br />
< input type= "checkbox" name= "sport"/>排球<br />
< input type= "checkbox" name= "sport"/>羽毛球<br/>
< input type= "checkbox" name= "sport"/>乒乓球<br/>
< p>最多选择三项</p>

var sportSelect = document.getElementsByName('sport' ),
              maxNums   = 3;
           for(var i in sportSelect){
              sportSelect[i]. onclick = function (){
                 var _sportSelect = document.getElementsByName('sport' ),
                   cNums      = 0;
                 for(var i in _sportSelect){
                   if(i == 'length') break ;
                   if(_sportSelect[i].checked){
                      cNums ++;
                   }
                }
                 if(cNums > maxNums){
                   this.checked = false;
                    alert('最多只能选择三项');
                }
              }
           }