Hi this is my example layout on mobile device.
https://codesandbox.io/s/cocky-dust-xuurof?file=/src/styles.css
On the wider screen the third element must be the same width like a second yellow element. The second and the third element. The second and third items together are the same height as the first item. The html structure cannot change, I need use Flexbox, not css grid.
.App {
  font-family: sans-serif;
  text-align: center;
  display: flex;
  border: 2px solid black;
  flex-wrap: wrap;
}
.one {
  background-color: red;
  flex-basis: 120px;
  height: 80px;
}
.two {
  background-color: yellow;
  flex-grow: 1;
}
.three {
  background-color: green;
  height: 30px;
  flex-basis: 100%;
}
@media only screen and (min-width: 500px) {
  .one {}
  .two {}
  .three {}
}<div class="App">
  <div class="one">1</div>
  <div class="two">2</div>
  <div class="three">3</div>
</div> 
     
    