<style>
.example3{
    /*コレ*/border-radius: 50%;
    display: flex;
    justify-content: center;
    align-items: center;
    width: 200px;
    height: 200px;
    background: #6fa1ff;
    color: #FFF;
    text-decoration: none;
    text-align: center;
    margin: 10px 0;
}
</style>
<a href="#" class="example3">丸ボタン</a>