#wrapper {
    display: flex;
}

#sectionA {
    width: 33%;
    border: 1px solid #000;
    height: 600px;
}

#sectionB {
    width: 33%;
    border: 1px solid #000;
    height: 600px;
}

#sectionC {
    width: 33%;
    height: 600px;
    border: 1px solid #000;
}