JSFiddle - React, Tailwind, and code Playground

HTML

<nav>
    <span>1</span>
    <span>2</span>
    <span>3</span>
</nav>
<div id="container">
</div>

CSS

nav {
    display: table;
    width: 500px;
}
#container {
    height: 400px;
    width: 500px;
    background: #5795E5;
    border-radius: 0 0 25px 25px;
}
nav span {
    display: table-cell;
    background: #46BA50;
    text-align: center;
    line-height: 55px;
    border-right: 1px solid #CBECCD;
}
nav span:first-child {
    border-radius: 25px 0 0 0;
}
nav span:last-child {
    border-radius: 0 25px 0 0;
}