Fill remaining width using flex

<div class="container">
<div class="flexible">test</div>
<div class="fixed">test</div>
<div class="fixed">test</div>
</div>

.container {
  display: flex;
}

.fixed {
  flex: 0 0 100px;
  background-color: red;
}

.flexible {
  flex: 1
}