我想实现无限321的循环，if语句怎么改成for循环

var num=0;

function numCount(){

     document.getElementById('txt').value=num;

    if(num<3){

         num=num+1;

        setTimeout("numCount()",1000);

    }else{

        num=0;

        setTimeout("numCount()",1000);

}

 }