why my card is not getting aligned horizontally

here is the code for the card section. The cards get down added vertically when added i want them to be horizontally aligned , 3 cards in one row . please help. Thanks a lot

<div class="container">
    <div class="row">

      <div class="col-md-4">
        <div class="card" style="width: 18rem;">
          <img class="card-img-top" src="..." alt="Card image cap">
          <div class="card-body">
            <h5 class="card-title">Card title</h5>
            <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
          </div>
          <ul class="list-group list-group-flush">
            <li class="list-group-item">Cras justo odio</li>
            <li class="list-group-item">Dapibus ac facilisis in</li>
            <li class="list-group-item">Vestibulum at eros</li>
          </ul>
          <div class="card-body">
            <a href="#" class="card-link">Card link</a>
            <a href="#" class="card-link">Another link</a>
           
            <div class="col-md-4">
            <div class="card" style="width: 18rem;">
              <img class="card-img-top" src="..." alt="Card image cap">
              <div class="card-body">
                <h5 class="card-title">Card title</h5>
                <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
              </div>
              <ul class="list-group list-group-flush">
                <li class="list-group-item">Cras justo odio</li>
                <li class="list-group-item">Dapibus ac facilisis in</li>
                <li class="list-group-item">Vestibulum at eros</li>
              </ul>
              <div class="card-body">
                <a href="#" class="card-link">Card link</a>
                <a href="#" class="card-link">Another link</a>
              </div>
            </div>
             
          </div>
        </div>
        </div>
      </div>
    </div>
  </div>

<div class="container">
<div class="row">
<div class="col-sm-4">
        <div class="card" style="width: 18rem;">
          <img class="card-img-top" src="..." alt="Card image cap">
          <div class="card-body">
            <h5 class="card-title">Card title</h5>
            <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
          </div>
          <ul class="list-group list-group-flush">
            <li class="list-group-item">Cras justo odio</li>
            <li class="list-group-item">Dapibus ac facilisis in</li>
            <li class="list-group-item">Vestibulum at eros</li>
          </ul>
          <div>
            <a href="#" class="card-link">Card link</a>
            <a href="#" class="card-link">Another link</a>
          </div>
    </div>
  </div>
  <div class="col-sm-4">
    <div class="card" style="width: 18rem;">
      <img class="card-img-top" src="..." alt="Card image cap">
      <div class="card-body">
        <h5 class="card-title">Card title</h5>
        <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
      </div>
      <ul class="list-group list-group-flush">
        <li class="list-group-item">Cras justo odio</li>
        <li class="list-group-item">Dapibus ac facilisis in</li>
        <li class="list-group-item">Vestibulum at eros</li>
      </ul>
      <div>
        <a href="#" class="card-link">Card link</a>
        <a href="#" class="card-link">Another link</a>
      </div>                 
    </div>
  </div>
       
        <div class="col-sm-4">
        <div class="card" style="width: 18rem;">
          <img class="card-img-top" src="..." alt="Card image cap">
          <div class="card-body">
            <h5 class="card-title">Card title</h5>
            <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
          </div>
          <ul class="list-group list-group-flush">
            <li class="list-group-item">Cras justo odio</li>
            <li class="list-group-item">Dapibus ac facilisis in</li>
            <li class="list-group-item">Vestibulum at eros</li>
          </ul>
          <div>
            <a href="#" class="card-link">Card link</a>
            <a href="#" class="card-link">Another link</a>
          </div>
    </div>
  </div>
</div>

Here is the structure for alignment.