var obj_table1 = document.getElementById("table1的id");
var obj_table2 = document.getElementById("table2的id");
var obj_table3 = document.getElementById("table3的id");obj_table1.style.pixelHeight = obj_table2.clientHeight - obj_table3.clientHeight;
var obj_table2 = document.getElementById("table2的id");
var obj_table3 = document.getElementById("table3的id");obj_table1.style.pixelHeight = obj_table2.clientHeight - obj_table3.clientHeight;
<tr>
<td> T1</td>
</tr>
</table><br /><table id="t2" width="400" border="1" cellspacing="0" cellpadding="0">
<tr>
<td> T2</td>
</tr>
<tr>
<td> </td>
</tr>
<tr>
<td> </td>
</tr>
</table><br /><table id="t3" width="400" border="1" cellspacing="0" cellpadding="0">
<tr>
<td> T3</td>
<td> </td>
</tr>
</table>
<script language="javascript" type="text/javascript">
function $()
{
var h2=getId("t2").clientHeight;
var h3=getId("t3").clientHeight;
var newHeight=parseInt(h2)-parseInt(h3)
if(newHeight>0)
getId("t1").style.height=newHeight+"PX";//支持IE、FF的写法
}
function getId(id)
{return document.getElementById(id);}
</script>
<input name="" type="button" onClick="$()" value="测试" />