JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container clear">
    <button class="btn" type="button">Button 1</button>
</div>
 
<div class="container clear">
    <button class="btn" type="button">Button 1</button>
    <button class="btn" type="button">Button 2</button>
</div>

<div class="container clear">
    <button class="btn" type="button">Button 1</button>
    <button class="btn" type="button">Button 2</button>
    <button class="btn" type="button">Button 3</button>
</div>

CSS

* {
    box-sizing: border-box;
}

.clear:before,
.clear:after {
    content: " ";
    display: table;
}

.clear:after {
    clear: both;
}

.container {
    margin-bottom: 15px;
}

.btn {
    display: inline-block;
    float: left;
    margin: 0;
    font-size: 14px;
    padding: 15px;
    border: 1px solid red;
    vertical-align: middle;
}

.container .btn:nth-child(1):nth-last-child(1) {
    width: 100%;
}

.container .btn:nth-child(1):nth-last-child(2),
.container .btn:nth-child(2):nth-last-child(1) {
    width: 50%;
}

.container .btn:nth-child(1):nth-last-child(3),
.container .btn:nth-child(2),
.container .btn:nth-child(3):nth-last-child(1) {
    width: 33.333333%;
}