JSFiddle - React, Tailwind, and code Playground

HTML

<div class="head"></div>
<div class="side"></div>
<div class="cont"></div>

CSS

.head {
  float:left;
  width: calc(100% - 100px);
  height: 100px;
  background: red;
}
.side {
  float:right;
  width: calc(100px);
  height: 200px;
  background: green;
}
.cont {
  float:left;
  width: calc(100% - 100px);
  height: 100px;
  background: blue;
}

@media screen and (max-width:600px) {
  .head,
  .side,
  .cont {
    float:none;
    width: auto
  }

}