looking now for some kind of manual
Did you read the Documentation on their website?
At it's most basic, copy & paste the following code into a new, blank document to test.
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Fancybox 3 Demo</title>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<!--fancybox latest minified CSS-->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/fancybox/3.0.47/jquery.fancybox.min.css"/>
<style>
.center-block {float:none; margin:0 auto; width: 80%;}
</style>
</head>
<body>
<!--fancybox gallery-->
<div class="center-block">
<a data-fancybox="gallery" href="https://dummyimage.com/500x325/000/fff.jpg&text=Image+1" alt="fullsize image"><img src="http://dummyimage.com/175x114" alt="smallsize"></a>
<a data-fancybox="gallery" href="https://dummyimage.com/500x325/fff/000.jpg&text=Image+2" alt="fullsize image"><img src="http://dummyimage.com/175x114" alt="smallsize"></a>
<a data-fancybox="gallery" href="https://dummyimage.com/500x325/000/fff.jpg&text=Image+3" alt="fullsize image"><img src="http://dummyimage.com/175x114" alt="smallsize"></a>
<a data-fancybox="gallery" href="https://dummyimage.com/500x325/fff/000.jpg&text=Image+4" alt="fullsize image"><img src="http://dummyimage.com/175x114" alt="smallsize"></a>
</div>
<!--jquery latest minified js-->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<!--fancybox latest minifed JS-->
<script src="https://cdnjs.cloudflare.com/ajax/libs/fancybox/3.0.47/jquery.fancybox.min.js"></script>
</body>
</html>