How does one get an order confirmation email to fire immediately in Commerce 3?
There are a couple of options here.

Number 1

If you have the config setting runQueueAutomatically set to false you could call the controller action queue/run via ajax from your success page. This would trigger the queue to run.

Number 2

You can use the EVENT_AFTER_COMPLETE_ORDER event to run some custom code, in a module or plugin. In that code you can include Craft::$app-&gt;getQueue()-&gt;run() this will also trigger the queue to run. e.g. 

Event::on(Order::class, Order::EVENT_AFTER_COMPLETE_ORDER, function(Event $e) {
    Craft::$app-&gt;getQueue()-&gt;run();
});