Well, in my last comment, I said the GTX 1050 is anemic. However, I didn’t suggest a specific video card for your application.
Your new GTX 1060 6GB is an improvement. It is a capable card for running XP11 at medium-high settings for a single display. But like I said, three monitors setup is highly GPU-challenged, even to a high-end video card. Flying over high density scenery, CPU is also important.
My guess is the GTX 1060 is powerful enough for this job only if you are willing to settle for low rendering settings (Visual Effects, Texture and Antialiasing). You can run Windows’ Task Manager to observe the CPU and GPU loading. I suspect the bottleneck to be the GPU.
You may find this video from Michael Brown informative. It gives an interesting graphical comparison on various GPUs and various resolutions have on FPS.
https://www.youtube.com/watch?v=vZESwoaZWDo