JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
    <div class="item">1</div>
    <div class="item">2</div>
    <div class="item">3</div>
    <div class="item">4</div>
    <div class="item">5</div>
    <div class="item">6</div>
    <div class="item">7</div>
    <div class="item">8</div>
    <div class="item">9</div>
    <div class="item">10</div>
    <div class="item">11</div>
    <div class="item">12</div>
</div>

CSS

#main .item {
    height: 100px;
    width:33.33%;
    float:left;
    text-align: center;
}
#main .item:nth-child(3n+1) {
    background: red;
}
#main .item:nth-child(3n+3) {
    background: yellow;
}
#main .item:nth-child(3n+2) {
    background:lightblue;
}
#main .item:nth-child(3n+1),
#main .item:nth-child(3n+2) {
    width: calc(33.33% - 20px);
    margin-right: 20px;
}