3.css

body {
    margin: 0px;
}

.parent {
    display: flex;
    background-color: yellow;
}

.one {
    width: 50px;
    background-color: red;
    height: 100px;
    flex-grow: 1;
}

.two {
    width: 50px;
    background-color: blue;
    height: 100px;
    flex-grow: 2;

}

.three {
    width: 50px;
    background-color: green;
    height: 100px;
    flex-grow: 3;

}