The case fans are for moving air through the case so that the inside temp of the case can stay cool and the fans on the devices can have cllo fresh air to blow onto the heatsinks to cool the devices. A usual configuration id to have the front fan blowing in and the rear and top fan blowing out. If you want to add a fan then you can add one on the side panel blowing air in. So that would give you two in and two out.
The side panel can accomodate two 120mm or one200mm and then there's the bottom of the case where you can put a fan also , but usually there are too many cables in the way of a bottom fan.
you should have the fans attached to a fan controller so that you have control of them and also control of the noise level. Since you have a 120mm and two 200mm then you can add a 120mm to have an equal amount of sizes of fans. If you put a fan controller then you could put a larger 200mm fan in the side panel since you would have control of it and have the same amount of air going in as going ut.