In my opinion, two intakes and one exhaust is the optimum setup if you want to keep a clean case.
When all the intake comes from one source and is filtered, your case will stay clean.
One exhaust is ok to direct the airflow past the cpu cooler and out the case.
Beware of using a second fan; it is counter productive.
When you do, it will tend to draw in unfiltered air from other openings.
Your parts are not hot so you need nothing more.