<script language="javascript">
function check(){
s.disabled=(t1.value==""||t2.value==""||t3.value=="")
}
</script>
<input name="t1" onpropertychange="check()" />
<input name="t2" onpropertychange="check()" />
<input name="t3" onpropertychange="check()" />
<input type="submit" name="s" disabled="desabled" />