Bootstrap-framework - col-sm - grid for small screens doesn't work
To solve the problem when the width is less than 576px, you should apply the col-4 style.
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css" integrity="sha384-Vkoo8x4CGsO3+Hhxv8T/Q5PaXtkKtu6ug5TOeNV6gBiFeWPGFN9MuhOf23Q9Ifjh" crossorigin="anonymous">
<div class="container">
<div class="row">
<div class="col-4 col-sm-4 col-md-4 col-lg-4 col-xl-4">
a
</div>
<div class="col-4 col-sm-4 col-md-4 col-lg-6 col-xl-6">
b
</div>
<div class="col-4 col-sm-4 col-md-4 col-lg-2 col-xl-2">
c
</div>
</div>
</div>