Loading Multiple On The Same Page?

I have a total of three different JavaScripts that need to be loaded when the user opens the page; let's call them script_a.js, script_b.js, and script_c.js

script_a.js and script_b.js each need to be loaded on every page; however, they do very different things (one displays text, the other creates buttons), and in the interest of readability and ease of editing later, I want to keep them separate.

script_c.js only needs to be loaded on one page; there will be a few variants (script_c2, script_c3, etc), but again, each of them only needs to be loaded on a single page.

The scripts are also too large and unwieldy for me to put them directly into the HTML (basically because I have to do a lot of different switches, and conditionals, and output formatting).

At present, the only way I know how to load a JavaScript is to use window.onload, so obviously I'm getting into the issue of competing onLoads.