<div class="input-group mb-3">
  <div class="input-group-prepend">
    <label class="input-group-text" for="inputGroupSelect01">Options</label>
  </div>
  <select class="custom-select" id="inputGroupSelect01">
    <option selected>Choose...</option>
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="3">Three</option>
  </select>
</div>

<div class="input-group mb-3">
  <select class="custom-select" id="inputGroupSelect02">
    <option selected>Choose...</option>
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="3">Three</option>
  </select>
  <div class="input-group-append">
    <label class="input-group-text" for="inputGroupSelect02">Options</label>
  </div>
</div>

<div class="input-group mb-3">
  <div class="input-group-prepend">
    <button class="btn btn-outline-secondary" type="button">Button</button>
  </div>
  <select class="custom-select" id="inputGroupSelect03" aria-label="Example select with button addon">
    <option selected>Choose...</option>
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="3">Three</option>
  </select>
</div>

<div class="input-group">
  <select class="custom-select" id="inputGroupSelect04" aria-label="Example select with button addon">
    <option selected>Choose...</option>
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="3">Three</option>
  </select>
  <div class="input-group-append">
    <button class="btn btn-outline-secondary" type="button">Button</button>
  </div>
</div>