[Vlasenko Fedor]

<input type="text" onkeyup="send.disabled = this.value ? false : 'disabled';" id="input" /><br />
<input type="submit" id="send" disabled="disabled"/>

<form>
<input type="text" onkeyup="check();" id="input1" /><br />
<input type="text" onkeyup="check();" id="input2" /><br />
<input type="text" onkeyup="check();" id="input3" /><br />
<input type="submit" id="send" disabled="disabled"/>
</form>
  <script>
function check() {
  var inp1 = document.getElementById('input1'),
      inp2 = document.getElementById('input2'),
      inp3 = document.getElementById('input3');
  document.getElementById('send').disabled = inp1.value && inp2.value && inp3.value ? false : "disabled";
}
</script>