This may seem like an odd question, but I am going to ask you what the URL is for the Bootstrap 3 website. I am doing this to help you avoid the heartache of downloading an old version. code example

Example: bootstrap getting started

<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">