<div>
  <span></span>
</div>
<div class="center">
  <span></span>
</div>
<div>
  <span></span>
</div>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  background-color: blue;
  display: flex;
  align-items: center;
  height: 100vh;
}
div {
  flex: 1 1 25%;
  height: 25vw;
  border-radius: 100%;
  background-color: springgreen;
  display: flex;
  justify-content: center;
  align-items: center;
}
.center {
  flex-basis: 50%;
  height: 50vw;
}