<div class="parent">
  <div class="child child-one">child</div>
  <div class="child child-two">child</div>
  <div class="child child-three">child</div>
</div>

* {
  box-sizing: border-box;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica,
    Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}

.parent {
  background-color: #001f3f;
  display: flex;
  padding: 20px;
}

.child {
  background-color: #0074d9;
  padding: 20px;
  flex-grow: 1;
  color: white;
}

.child-three {
  flex-grow: 3;
  background-color: #3d9970;
}