BigSmoke

<div class="wrap">
  <div class="box box-1">1</div>
  <div class="box box-2">2</div>
  <div class="box box-3">3</div>
</div>

html, body {
  height: 100%;
  margin: 0;
}

.wrap {
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  height: 100%;
}

.box {
  font-size: 20px;
  display: flex;
  justify-content: center;
  align-items: center;
  width: 70px;
  height: 70px;
  background-color: green;
}

.box-3 {
  margin-top: auto;
}

<div class="container">
  <aside></aside>
  <aside></aside>
  <main></main>
  <aside></aside>
  <aside></aside>
</div>

.container {
  width:900px;
  height:200px;
  display:flex;
  flex-direction:column;
  flex-wrap:wrap;
}
aside {
  width:300px;
  height:100px;
}
main {
  width:300px;
  height:200px;
}