Multiple monitors with different resolutions

I operate two monitors with a resolution of 1920x1080 pixels and one 4K-monitor with a MacPro. It is said to support 3 x 4K-monitors. Each time the images are refreshed the 4K image is set back to the size of the others. I have tried to use the HDMI-port or the display port as suggested by some. The outcome is the same.

By the way, with the display port, I can either run the monitor with the native resolution which is almost useless because you can use it as a display only. Or I can use it as a 1080p display giving an extremely bad resolution for text. If I select the native resolution, the pull down menus are almost illegible, and pop up windows etc. are so small that one can guess what they might depict.

I prefer to use SwitchResX and to select a reasonable resolution. But the main problem that all open windows on that screen are resized remains. I would welcome any idea helping change this. What else can I do except dumping my 4K monitor?