当三个下拉列表之和大于300时显示div

<select id="list1" onchange="calculateSum()">
  <option value="0">0</option>
  <option value="1">1</option>
  ...
</select>

<select id="list2" onchange="calculateSum()">
  <option value="0">0</option>
  <option value="1">1</option>
  ...
</select>

<select id="list3" onchange="calculateSum()">
  <option value="0">0</option>
  <option value="1">1</option>
  ...
</select>

<div id="result" style="display:none;">
  这是显示的div内容。
</div>

function calculateSum() {
  var list1 = document.getElementById("list1").value;
  var list2 = document.getElementById("list2").value;
  var list3 = document.getElementById("list3").value;
  var sum = parseInt(list1) + parseInt(list2) + parseInt(list3);

  if (sum > 300) {
    document.getElementById("result").style.display = "block";
  } else {
    document.getElementById("result").style.display = "none";
  }
}