How do I get Roboto Google Font to display normal font-style? [closed]

What you are experience is probably issue with something else. If you try and access this link:

https://fonts.googleapis.com/css?family=Roboto:300

Directly, you can see that all the font families have a normal style. You might have another CSS rule overriding them. Check for that, and also refresh your browser’s cache.

The second note is, you are only registering the styles. You have to also enqueue them using wp_enqueue_style. So your code should be like this:

function google_fonts() {
    $query_args = array(
        'family' => 'Roboto:300'
    );
    wp_register_style( 'google_fonts', add_query_arg( $query_args, "https://fonts.googleapis.com/css" ), array(), null );
    wp_enqueue_style('google_fonts');
}

add_action('wp_enqueue_scripts', 'google_fonts');

Right now I’m guessing you are using some other enqueued google font.