Puoi fare cosi.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<table summary="">
<tr>
<td>fffffffff</td>
<td id="cella"></td>
<td>fffffffff</td>
</tr>
</table>
<script language="JavaScript" type="text/javascript">
<!--
document.getElementById('cella').innerText = '3'; 
//-->
</script>

</body>
</html>