JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div class="box left">w: 50px</div>
    <div class="box middle">width of this box is variable</div>
    <div class="box right"> width: 100%; should occupy remaining width of container</div>
</div>

CSS

#container { 
    width: 70%;
    border: 2px solid black;
    border-right: 2px dashed black;
    font-size: 0;
    white-space: nowrap;
}
    
.left {
    width: 75px;
    background-color: orangered;
}

.middle {
    background-color: lightgreen;
}

.right {
    width: 100%;
    background: rgba(0, 255, 255, .5);
    text-align: left !important;
}

.box {
    height: 50px;
    box-sizing: border-box;
    padding: 15px 5px;
    text-align: center;
    display: inline-block;
    font-size: 16px;
}