how to total up all drop down menu : i need help im new to this but i have 3 drop down menus how would i total them all up to prompt the screen if the total is over 10  in total 

i have add the code im using below

    <table align="center" width="360" border="0" cellspacing="0" cellpadding="0">
           <tr>
         <td width="270" align="right">N1:</td>
      <td width="270" align="right"><select name="N1" id="N1">
          <option value="0">0</option>
          <option value="1">1</option>
          <option value="2">2</option>
          <option value="3">3</option>
          <option value="4">4</option>
          <option value="5">5</option>
         </select></td>
         <td width="270" align="right">N2:</td>
      <td width="270" align="right"><select name="N2" id="N2">
          <option value="0">0</option>
          <option value="1">1</option>
          <option value="2">2</option>
          <option value="3">3</option>
          <option value="4">4</option>
          <option value="5">5</option>
         </select></td>
          </tr>
         <td width="370" align="right">N3:</td>
      <td width="270" align="right"><select name="N3" id="N3">
          <option value="0">0</option>
          <option value="1">1</option>
          <option value="2">2</option>
          <option value="3">3</option>
          <option value="4">4</option>
          <option value="5">5</option>
         
         </td> 
    
      </tr>
</table>  