.card-img-top {
width: 100%;
height: 15vw;
object-fit: cover;
}
<div class="card" style="width: 18rem;">
<img class="card-img-top" src="..." alt="Card image cap">
<div class="card-body">
<h5 class="card-title">Card title</h5>
<p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
<a href="#" class="btn btn-primary">Go somewhere</a>
</div>
</div>
<div class="card" style="width: 18rem;">
<img src="..." class="card-img-top" alt="...">
<div class="card-body">
<h5 class="card-title">Card title</h5>
<p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
<a href="#" class="btn btn-primary">Go somewhere</a>
</div>
</div>
<div class="card-group">
<div class="card">
<img src="..." class="card-img-top" alt="...">
<div class="card-body">
<h5 class="card-title">Card title</h5>
<p class="card-text">This is a wider card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
<p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
</div>
</div>
<div class="card">
<img src="..." class="card-img-top" alt="...">
<div class="card-body">
<h5 class="card-title">Card title</h5>
<p class="card-text">This card has supporting text below as a natural lead-in to additional content.</p>
<p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
</div>
</div>
<div class="card">
<img src="..." class="card-img-top" alt="...">
<div class="card-body">
<h5 class="card-title">Card title</h5>
<p class="card-text">This is a wider card with supporting text below as a natural lead-in to additional content. This card has even longer content than the first to show that equal height action.</p>
<p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
</div>
</div>
</div>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
<body>
<div class="container">
<div class="row">
<div class="col-4">
<div class="card">1</div>
</div>
<div class="col-4">
<div class="card">1</div>
</div>
<div class="col-4">
<div class="card">1</div>
</div>
</div>
</div>
</body>
</html>