如果选择了下拉列表中的第三个选项，则文本框将被禁用，并会出现一个新框

<select id="dropdown">
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

<input type="text" id="textbox">

var dropdown = document.getElementById("dropdown");
var textbox = document.getElementById("textbox");

dropdown.addEventListener("change", function() {
  if (dropdown.value === "option3") {
    textbox.disabled = true;
    createNewTextbox();
  } else {
    textbox.disabled = false;
  }
});

function createNewTextbox() {
  var newTextbox = document.createElement("input");
  newTextbox.type = "text";
  newTextbox.id = "newTextbox";
  document.body.appendChild(newTextbox);
}