SOURCE

<div class="wrap">
  <div class="B B1">B1</div>
  <div class="B B2">B2</div>
  <div class="B B3">B3</div>
</div>

.wrap {
  border: 1px solid red;
  height: 500px;
  display: flex;
  width: 500px;
}

.wrap .B {
  width: 100px;
  height: 100px;
  box-sizing: border-box;
}

.B1 {
  background-color:rgba(255,255,0,.5);
  flex-grow: 1;
}

.B2{
    background-color:rgba(255,0,255,.5);
}
.B3{
    background-color:rgba(0,255,255,.5);
}