The quickest way is to copy paste the code from the 3 files into one, let's call it combined.js
Then in your theme, replace the head_script function with this:
public function head_script()
{
if (isset($this->content['script'])) {
foreach ($this->content['script'] as $scriptline) {
if (!preg_match('#<script src=".*/qa-content/jquery-#', $scriptline) && !preg_match('#<script src=".*/qa-content/qa-global.js#', $scriptline))
$this->output_raw($scriptline);
}
$this->output_raw('<script src="./qa-content/combined.js"></script>');
}
}
(Note I haven't tested this)
FYI it's difficult to do this in core because the scripts come from different places. jQuery has always been kept separate so that people could replace it in custom themes (e.g. with a version from a CDN). The qa-global.js is already combined from previous different scripts (qa-question.js, qa-ask.js...). And the Snow JS is specific to that theme - if you change theme you'd be adding unnecessary JS.
