This simple bash command could save you if you doesnt have time to configure tool such as gulp, grunts or others, only works in *nix family OS (e.g linux, unix, mac osx)
cat jquery.min.js bootstrap.min.js custom.min.js > all.js
cat jquery.min.css bootstrap.min.css custom.min.css > all.css
now you can just load your asset files in html with
<html>
<head>
<title>Your Title Page</title>
<link rel="stylesheet" href="all.css">
</head>
<body>
<!-- your body html -->
<script src="all.js"></script>
<body>
</html>
Hope it helps. Have a great day!