.box {
    display: flex;
    gap: .5rem;
}

    .box.fs > :nth-child(1), /*fill start*/
    .box.fe > :nth-child(2) /*fill end*/ {
        -ms-flex: 1;
        -webkit-flex: 1;
        flex: 1;
    }

    .box.mid {
        -webkit-align-items: center;
        align-items: center;
    }