Use the .flex-*- column class in Bootstrap to show flex items vertically on different screen sizes:
Set it vertically on small, medium, large, etc screen size. Let us see it on small and medium screen size:
Small Screen Size
<p>Flex on different screen size (Small)</p>
<div class="d-flex flex-sm-column mb-3">
<div class="p-2 bg-warning">One</div>
<div class="p-2 bg-danger">Two</div>
<div class="p-2 bg-secondary">Three</div>
</div>
Medium Screen Size
<p>Flex on different screen size (Medium)</p>
<div class="d-flex flex-md-column mb-3">
<div class="p-2 bg-warning">One</div>
<div class="p-2 bg-danger">Two</div>
<div class="p-2 bg-secondary">Three</div>
</div>
The following is an example implementing what I have shown above the usage of flex-*-column:
Live Demo
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
<h2>Example</h2>
<p>Default </p>
<div class="d-flex flex-column mb-3">
<div class="p-2 bg-primary">One</div>
<div class="p-2 bg-secondary">Two</div>
<div class="p-2 bg-info">Three</div>
</div>
<p>Flex on different screen size (Small)</p>
<div class="d-flex flex-sm-column mb-3">
<div class="p-2 bg-warning">One</div>
<div class="p-2 bg-danger">Two</div>
<div class="p-2 bg-secondary">Three</div>
</div>
<p>Flex on different screen size (Medium)</p>
<div class="d-flex flex-md-column mb-3">
<div class="p-2 bg-warning">One</div>
<div class="p-2 bg-danger">Two</div>
<div class="p-2 bg-secondary">Three</div>
</div>
<p>Flex on different screen size (Large)</p>
<div class="d-flex flex-lg-column mb-3">
<div class="p-2 bg-primary">One</div>
<div class="p-2 bg-secondary">Two</div>
<div class="p-2 bg-danger">Three</div>
</div>
</div>
</body>
</html>