<div class="wrapper">
    <div class="box item1">1</div>
    <div class="box item2">2</div>
    <div class="box item3">3</div>
</div>

.wrapper {
  display: grid;
  gap: 1em;
  grid-template-columns: repeat(2, 1fr);
}

.box {
  border: 1px solid red;
  background-color: gray;
  padding: 2em;
}

.item3 {
  grid-column: 1 / 3;
  place-self: center;
  width: 50%;
}