SOURCE

<div id="app">
    <div class="class1 class">1</div>
    <div class="class2 class22">2</div>
    <div class="class3 class33">3</div>
</div>

#app {
    display: flex;
}

.class {
    flex-basis: 150px;
}

.class1 {
    background: red;
}

.class11 {
    flex-grow: 1;
}

.class2 {
    background: blue;
}

.class22 {
    flex-grow: 2;
}

.class3 {
    background: green;
}

.class33 {
    flex-grow: 3;
}