Your parts are not particularly heat intensive.
I7-8700 is not overclockable, so a standard tower type air cooler with a 120mm fan is all you need.
Scythe kotetsu is one of the most efficient and quietest if you have 160mm available.
Here is a review:
http://www.silentpcreview.com/article1391-page1.html
If you have only 145mm available, the cryorig H7 is also good.
Your current arrangement of two front 120mm intakes will supply all the cooling your rig requires.
The single 120mm rear exhaust is useful only for directing the airflow.
Adding fans can only increase noise.
Do not do that without a good reason.
If you want to reduce noise, you can undervolt the fans to make them run slower and still get adequate cooling.
Ultimately, it is the higher rpm that makes noise.
There are secondary effects such as quality of the sound which can be reduced with better quality fans.
Your rig should be all but inaudible as is if you use a decent cpu cooler instead of the stock intel cooler.