Prova cosi':
codice:
<script>
function controlla(obj) {
  rv = false;
  for (var i=1;i<=3;i++) {
    cp = document.getElementById("campo0"+i);
	rv = rv || (cp!=obj && cp.value.length>0);
  }
  if (rv) obj.blur();
}
</script>
<body>
<input name="campo01" type="text" class="questo_input" id="campo01" onfocus="controlla(this)">
<input name="campo02" type="text" class="questo_input" id="campo02" onfocus="controlla(this)">
<input name="campo03" type="text" class="questo_input" id="campo03" onfocus="controlla(this)">
</body>
ciao