idem, un minimo piu' evoluto
codice:
function enableNext(el){
	var nextEl=false;
	var toDisable=false;
	var theForm=el.form;

	for(var k=1;k<=3;k++){
		if(nextEl){
			theForm.elements['text'+k].disabled=toDisable;
			if(toDisable){theForm.elements['text'+k].value='';}
			else nextEl=false;
		}
		if(el.name=='text'+k){
			if(el.value=='') toDisable=true;
			nextEl=true;
		}
	}
}
codice:
<form>
<input type="text" name="text1" onkeyup="enableNext(this)" />

<input type="text" name="text2" onkeyup="enableNext(this)" disabled="disabled" />

<input type="text" name="text3" disabled="disabled" />

</form>