With a huge 120mm fan at the back and don't forget the fan/fans on your power supply you won't need a side fan. A fan at the front of your case drawing air in is a good idea though.If the fans are high at the back its better to have a low fan at the front

Ideally you want cool air coming into from the front of your case travelling across your entire system and exiting at the rear. Start putting side fans in and your going to get turbulant airflow which will not be as effiecent giving you more noise for less cooling.