<form oninput="result.value = parseInt(input1.value) + parseInt(input2.value) + parseInt(input3.value);">
  <input type="number" name="input1" value="5"> + 
  <input type="number" name="input2" value="10"> + 
  <input type="number" name="input3" value="15"> =
  <output name="result" for="input1 input2 input3"></output>
</form>