下面的代码将循环遍历数组，创建一个选项，然后使用.country将其附加到所有选择。这个解决方案使用jquery。

var country_arr = new Array("Afghanistan", "Albania", "Algeria", "American Samoa");

country_arr.forEach( function(obj) {
  var temp_country = new Option(obj, obj);
  $(temp_country).html(obj);
  $("select.country").append(temp_country);
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<select class="country" name ="country"></select>
<select class="country" name ="country"></select>
<select class="country" name ="country"></select> </div>

您需要迭代使用document.getElementsByClassName("country")获得的HTMLCollection元素：

var country_arr = new Array("Afghanistan", "Albania", "Algeria", "American Samoa" /*, ... */ );

function country() {
  var x = "<option disabled>Select Country</option>";
  
  for (var i = 0; i < country_arr.length; i++) {
    var node = country_arr[i];
    x += "<option value='" + country_arr[i] + "'> " + country_arr[i] + " </option>"
  }
  
  let list = document.getElementsByClassName("country");
  for (let item of list) {
    item.innerHTML = x;
  }

}

country();

<div style='text-align:center;'>
  <select class="country" name="country"></select>
  <select class="country" name="country"></select>
  <select class="country" name="country"></select>
</div>