JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
    <div id="centerWrapper">
        <div id="firstChild"></div>
        <div id="secondChild"></div>
    </div>
    <div id="thirdChild"></div>
</div>

CSS

*,
*:before,
*:after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html,
body,
#parent {
    width: 100%;
    height: 100%;
}

#centerWrapper {
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
}

#centerWrapper * {
    width: 100px;
    height: 100px;
}

#firstChild {
    background-color: brown;
}

#secondChild {
    background-color: lightblue;
}

#thirdChild {
    background-color: green;
    height: 100px;
    width: 100px;
    margin: auto;
    position: absolute;
    left: 0;
    right: 0;
    bottom: 0;
}

#parent {
    position: relative;
}