Ciao floyd,
innanzitutto grazie dell'aiuto, ho scritto come mi dicevi, ma purtroppo però se nelle funzioni delle addizioni continuo a saltare un campo questo non viene ancora considerato come nullo ma continuo ad avere come risultato zero. Ti riporto lo script modificato in base alle tue indicazioni.

codice:
<script language = "Javascript">


function taLimit() {
	var taObj=event.srcElement;
	if (taObj.value.length==taObj.maxLength*1) return false;
}

function taCount(visCnt) { 
	var taObj=event.srcElement;
	if (taObj.value.length>taObj.maxLength*1) taObj.value=taObj.value.substring(0,taObj.maxLength*1);
	if (visCnt) visCnt.innerText=taObj.maxLength-taObj.value.length;
}

function division1()
{
    var div1, div2, resultwe;

    div1 = parseFloat(document.form1.divis1.value);
    div2 = parseFloat(document.form1.divisore.value);
    resultwe = div1 / div2;

    document.form1.txtNumber1.value = resultwe;
}

function division2()
{
    var div3, div4, resultwr;

    div3 = parseFloat(document.form1.divis2.value);
    div4 = parseFloat(document.form1.divisore.value);
    resultwr = div3 / div4;

    document.form1.txtNumber2.value = resultwr;
}

function division3()
{
    var div5, div6, resultqw;

    div3 = parseFloat(document.form1.divis3.value);
    div4 = parseFloat(document.form1.divisore.value);
    resultqw = div3 / div4;

    document.form1.txtNumber3.value = resultqw;
}


function addition()
{

   var number1, number2, number3, result;
	
	var txtNumber1 = document.form1.txtNumber1.value;
	var txtNumber2 = document.form1.txtNumber2.value;
	var txtNumber3 = document.form1.txtNumber3.value;
	
    number1 = (isNaN(txtNumber1))? 0 : parseFloat(document.form1.txtNumber1.value);
    number2 = (isNaN(txtNumber2))? 0 : parseFloat(document.form1.txtNumber2.value);
	number3 = (isNaN(txtNumber3))? 0 : parseFloat(document.form1.txtNumber3.value);
    result =(number1 + number2 + number3);

    document.form1.txtResult.value=result;
}

function addition2()
{
    var number4, number5, number6, resultz;
	
	var divis1 = document.form1.divis1.value;
	var divis2 = document.form1.divis2.value;
	var divis3 = document.form1.divis3.value;

    number4 = (isNaN(divis1))? 0 : parseFloat(document.form1.divis1.value);
    number5 = (isNaN(divis2))? 0 : parseFloat(document.form1.divis2.value);
	number6 = (isNaN(divis3))? 0 : parseFloat(document.form1.divis3.value);
    resultz =(number4 + number5 + number6);

    document.form1.txtResultz.value=resultz;
}
</script>