the side fan(s) are intended to provide cooling for the GPU(s) and motherboard and are normally configured as intakes. Heat rises and is normally exhausted out the rear and top. However, there is no definitive "right" way to configure your fans. Having three intakes and one exhaust would probably make your setup a positive pressure situation, meaning that there would be more air going in the intakes than being exhausted, meaning the excess will vent through other openings in your case. This is not necessarily a bad thing, helps keep down dust build-up inside the case. However, you can find the pros and cons of positive pressure vs negative pressure in many online articles. The main point is, use whatever works best for you. Assuming you have a way of monitoring your temperatures, try different setups and compare the results then decide what works best for your case.