bootstrap中对齐按钮组的等宽下拉菜单

<div class="btn-group">
  <button type="button" class="btn btn-primary">按钮1</button>
  <button type="button" class="btn btn-primary">按钮2</button>
  <div class="btn-group">
    <button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
      下拉按钮
    </button>
    <div class="dropdown-menu">
      <a class="dropdown-item" href="#">菜单项1</a>
      <a class="dropdown-item" href="#">菜单项2</a>
      <a class="dropdown-item" href="#">菜单项3</a>
    </div>
  </div>
</div>