Adding another radiator will do nothing to improve your temps. With 2 360mm radiators in your loop you are already well past the point of diminishing returns for adding radiators as you have more than enough thermal dissipation capacity to cool both CPU and GPU. What is your ambient temperature in your room? Higher ambient temperature will lead to higher load temperatures. Also if you don't have adequate space where your PC is to move the hot air from the exhaust so that your PC won't recirculate the warm air, like if your PC is in a corner or under a desk that could lead to warmer temps. What could also affect your temps is a poor mount of the waterblock with uneven mounting pressure. You could also have a concave or convex IHS on your CPU which only lapping will make it flat.
 
Could be many different things, but also don't take open air test bench reviews as gospel for what you should and should not be getting for temperatures. Every setup and configuration poses its own unique challenge and unless you are also running a open bench setup, temps reviewers get should be largely ignored. 

75C under load on a 5950X is fine.
The 5950X will run to 90C before it begins throttling.
Adding more rad isn't going to do anything for you.
Push-Pull might lower you a degree or two.  Max.
Of course, in push-pull, you can run the fans at lower RPM (and therefore quieter).