sample3.html

<!DOCTYPE html>
<html>
<head>
<title>Задание 3</title>
<style>
.biggest {
height: 800px;
width: 800px;
display: flex;
align-items: center;
justify-content: center;
background: blue;
}
.middle {
height: 500px;
width: 500px;
display: flex;
align-items: center;
justify-content: center;
background: yellow;
}
.smallest {
height: 200px;
width: 200px;
background: red;
}
</style>
</head>
<body>
<div class="biggest">
<div class="middle">
<div class="smallest">
</div>
</div>
</div>
</body>
</html>