Ideally, you would have two 140mm fans in front as intakes and one 120mm fan in the rear as exhaust.
Or, simply add a 140mm to the front as an added intake.
140mm fans turn slower and are quieter while moving more air.
Pay more attention to intake.
All the cooling air that comes in the front will exit SOMEWHERE, taking heat with it.
If you look carefully at a fan, there will usually be some molded arrows indicating the direction of rotation and airflow.
I will mark them with a black marker so They are more visible.
Commonly, the sticker side is the intake side.