selection {

  height:70%;width:100%;margin:30px 0;

  display:flex;justify-content:flex-end;

}

aside {

  height:50px;width:100%;

  position: sticky;

  top;0px;text-align:center;display:flex;align-items:center;justify-content:center;

}

<div id="container">

  <section>

    <aside>1</aside>

  </section>

  <section>

    <aside>2</aside>

  </section>

  <section>

    <aside>3</aside>

  </section>

</div>