.overlay {
    position: absolute;
    width: 100%;
    height: 100%;
    left: 0%;
    z-index: 999999999; /* A very high value to ensure it's above all other elements */
    background: #e10100;
}

.first {
    top: 0;
    /* No need for z-index for .first, as it's part of .overlay */
}

.second {
    top: 33.3%;
    /* No need for z-index for .second, as it's part of .overlay */
}

.third {
    top: 66.6%;
    /* No need for z-index for .third, as it's part of .overlay */
}