<select name="h1" onchange="var submit_button = document.getElementById('submit_button'); submit_button.disabled = (this.value == 'select3');">
<option value='select1'>select1</option>
<option value='select2'>select2</option>
<option value='select3'>select3</option>
</select>
<input id="submit_button" type="submit" value='отправить'>