Ops scusami, non avevo capito bene. Ti passo tutto lo script, completo, in realtà l'addizione è solo parte di uno script che segue anche alcune divisioni partendo da valori inseriti all'interno dello stesso campo.

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento senza titolo</title>
<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;

    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;

    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>
</head>

<body>
<form id="form1" name="form1" method="post" action="">
  <table width="300" border="0" cellspacing="0" cellpadding="0">
    <tr>
      <td width="100" height="30"></td>
      <td width="100" height="30"><div align="center">Divisore</div></td>
      <td width="100" height="30"><div align="center">
        <input name="divisore" type="text"    id="divisore"  size="8" />
      </div></td>
    </tr>
    <tr>
      <td width="100" height="30"><div align="center">1&deg; Numero </div></td>
      <td width="100" height="30">
        <div align="center">
          <input name="divis1" type="text"    id="divis1" onkeyup="division1();addition2()" size="8" />
      </div></td>
      <td width="100" height="30"><div align="center">
        <input name="txtNumber1" type="text"    id="txtNumber1" onchange="addition()" size="8" />
      </div></td>
    </tr>
    <tr>
      <td width="100" height="30"><div align="center">2&deg; Numero </div></td>
      <td width="100" height="30">
        <div align="center">
          <input name="divis2" type="text"    id="divis2" onkeyup="division2();addition2()" size="8" />
      </div></td>
      <td width="100" height="30"><div align="center">
        <input name="txtNumber2" type="text"    id="txtNumber2" onchange="addition()" size="8" />
      </div></td>
    </tr>
    <tr>
      <td width="100" height="30"><div align="center">3&deg; Numero </div></td>
      <td width="100" height="30">
        <div align="center">
          <input name="divis3" type="text"    id="divis3" onkeyup="division3();addition2()" size="8" />
      </div></td>
      <td width="100" height="30"><div align="center">
        <input name="txtNumber3" type="text"    id="txtNumber3" onchange="addition()" size="8" />
      </div></td>
    </tr>
    <tr>
      <td width="100" height="30"><div align="center">Risultati</div></td>
      <td width="100" height="30">
        <div align="center">
          <input name="txtResultz" type="text"    id="txtResultz"  size="8" />
      </div></td>
      <td width="100" height="30"><div align="center">
        <input name="txtResult" type="text"    id="txtResult"  size="8" />
      </div></td>
    </tr>
    <tr>
      <td width="100" height="30"></td>
      <td width="100" height="30"><div align="center"></div></td>
      <td width="100" height="30"><div align="center"></div></td>
    </tr>
  </table>
</form>
</body>
</html>