如何重叠多个div？

<style>
    .div1 {
        position: relative;
        background-color: red;
        width: 200px;
        height: 200px;
        z-index: 1;
    }

    .div2 {
        position: absolute;
        top: 50px;
        left: 50px;
        background-color: blue;
        width: 200px;
        height: 200px;
        z-index: 2;
    }

    .div3 {
        position: absolute;
        top: 100px;
        left: 100px;
        background-color: green;
        width: 200px;
        height: 200px;
        z-index: 3;
    }
</style>

<div class="div1"></div>
<div class="div2"></div>
<div class="div3"></div>