Hola, espero este codigo te sirva.
Código HTML:
<html>
<body>
<script>
function vacios(){
var c1=document.getElementById('c1').value;
var c2=document.getElementById('c2').value;
var c3=document.getElementById('c3').value;
if( (c1=="") || (c2=="") || (c3==""))
document.getElementById('ok').disabled=true;
else
document.getElementById('ok').disabled=false;
}
</script>
<form>
<input type='text' id='c1'>
<input type='text' id='c2'>
<input type='text' id='c3'>
<input type='submit' id='ok'>
</form>
<script>
setInterval("vacios()",500);
</script>
</body>
</html> suerte.