Index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
    <meta name="viewport" content="width=device-width, user-scalable=no">
    <title>ThankOST</title>
    <link rel="stylesheet" href="css/styles.css" />
</head>
<body>

    <div class="wrapper">
        <div class="flex-container">
            <div class="box one"></div>
            <div class="box two"></div>
            <div class="box three"></div>
        </div>
    </div>

</body>
</html>

CSS:

*{
    font-family: verdana;
    margin: 0;
}

body{
    background: #eee;
}

.wrapper{
    width: 100%;
    max-width: 960px;
    margin: 0 auto;
}

.flex-container{
    display: flex;
    background-color: #fff;
    flex-wrap: wrap;
    /*flex-wrap: wrap-reverse;*/
    /*flex-flow: column;*/
    /*flex-flow: column-reverse;*/
    flex-flow: row; /* column-reverse, row, row-reverse*/
    height: 600px;
    align-items: center; /*flex-end, flex-start*/ /* CROSS AXIS aligns the items in vertical center*/
    /*justify-content: center; !* MAIN AXIS flex-start, flex-end, space-between, space-around*!*/
}
.box{
    height: 100px;
    /*min-width: 200px;*/
    /*flex-basis: 200px;*/
    /*flex-grow: 1;*/
    flex: 0 0 100px;
    /*flex: 1 0 200px;*/
    /*1 - flex-grow*/
    /*0 - flex-shrink*/
    /*200px - flex-basis*/
}
.one{
    background: red;
    /*flex-grow: 2;*/
    /*flex-basis: 100px;*/
}
.two{
    background: blue;
    /*flex-grow: 3;*/
    /*flex-basis: 200px;*/
}
.three{
    background: green;
    /*flex-grow: 1;*/
    /*flex-basis: 300px;*/
}