<div class="treeview">

  <ul>

   <li class="d-flex flex-row"><a href="#"> One</a> <div class="dropdown mx-4">

  <button

    class="btn btn-primary btn-sm dropdown-toggle"

    type="button"

    data-mdb-toggle="dropdown"

    aria-expanded="false"

  >

  </button>

  <ul class="dropdown-menu"  >

    <li><a class="dropdown-item" href="#">Action</a></li>

    <li><a class="dropdown-item" href="#">Another action</a></li>

    <li><a class="dropdown-item" href="#">Something else here</a></li>

  </ul>

</div> </li> 

     <li class="d-flex flex-row"><a href="#"> Two</a>  <div class="dropdown mx-4">

  <button

    class="btn btn-primary btn-sm dropdown-toggle"

    type="button"

    data-mdb-toggle="dropdown"

    aria-expanded="false"

  >

  </button>

  <ul class="dropdown-menu"  >

    <li><a class="dropdown-item" href="#">Action</a></li>

    <li><a class="dropdown-item" href="#">Another action</a></li>

    <li><a class="dropdown-item" href="#">Something else here</a></li>

  </ul>

</div></li>

    <li>

      <a>Three</a>

      <ul class="show">

        <li class="d-flex flex-row">Second-one

        <div class="dropdown mx-4">

  <button

    class="btn btn-primary btn-sm dropdown-toggle"

    type="button"

    data-mdb-toggle="dropdown"

    aria-expanded="false"

  >

  </button>

  <ul class="dropdown-menu"  >

    <li><a class="dropdown-item" href="#">Action</a></li>

    <li><a class="dropdown-item" href="#">Another action</a></li>

    <li><a class="dropdown-item" href="#">Something else here</a></li>

  </ul>

</div>

        </li>

        <li>Second-two</li>

        <li>

          <a>Second-three</a>

          <ul>

            <li>

              <a>Third-one</a>

              <ul>

                <li>Fourth-one</li>

                <li>Fourth-two</li>

                <li>Fourth-three</li>

              </ul>

            </li>

            <li>Third-two</li>

            <li>

              <a>Third-three</a>

              <ul>

                <li>Fourth-one</li>

                <li>Fourth-two</li>

                <li>Fourth-three</li>

              </ul>

            </li>

          </ul>

        </li>

      </ul>

    </li>

  </ul>

</div>

dropdown-toggle {

  padding: 2px;

  width: 10px;

}

​