Bootstrap grid system not working - columns stack vertically

I am trying to learn Bootstrap. Would like to create simple 3 columns in 1 row.

My code looks like this (just copied it from example in Bootstrap Documentation).

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <!-- The above 3 meta tags *must* come first in the head; any other head content must come *after* these tags -->
  <title>Bootstrap 101 Template</title>

  <!-- Bootstrap -->
  <link href="css/bootstrap.min.css" rel="stylesheet">

  <!-- HTML5 shim and Respond.js for IE8 support of HTML5 elements and media queries -->
  <!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
  <!--[if lt IE 9]>
      <script src="https://oss.maxcdn.com/html5shiv/3.7.3/html5shiv.min.js"></script>
      <script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
      <![endif]-->
</head>

<body>

  <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
  <!-- Include all compiled plugins (below), or include individual files as needed -->
  <script src="js/bootstrap.min.js"></script>

  <div class="container">
    <div class="row">
      <div class="col-sm">
        One of three columns
      </div>
      <div class="col-sm">
        One of three columns
      </div>
      <div class="col-sm">
        One of three columns
      </div>
    </div>
  </div>
</body>

</html>

I tried with all kinds of sizes - "col-sm", "col-xs", etc, tried also "col", "col-different sizes", "col-different sizes-different numbers from 2 to 12" and the columns always stack vertically. None of the solutions on stackoverflow and the internet seem to work either.

Thanks in advance

Solution

Thank you guys for help.

The reason was not including Bootstrap's CSS properly.