<div class="parent">
  <div class="child">1</div>
  <div class="child">2</div>
  <div class="child">3</div>
</div>

.parent {
  display: flex;
  flex-wrap: wrap;
}

.child {
  flex-grow: 1;
}

.child:last-child {
  width: 100%;
}
  
/* --------- Non-flexbox related styles --------- */
.parent > div {
  margin: 2%;
  font-size: 60px;
  line-height: 130px;
  color: #ffe8c3;
  text-align: center;
  background-color: #eab768;
  border-radius: 10px;
  border: 10px solid #f5cc8b;
}

body {
  padding: 20px;
  background-color: #282c35;
  font-family: Verdana;
  font-weight: 600;
}