MBP, external screens go black when entering fullscreen

[goodfidelity]

I have 2 external monitor spaces (dual image, one monitor, dual cables) and one internal monitor space on my MBP 13" retina 2015.

If i choose to not use the function called "display has separate spaces" any windows that rests BETWEEN the screens will only be partially displayed.

If i change that to use this function, enabeling full screen mode on one monitor will make the others go black.

I want to know, are the developers working on this retarded or is this some kind of feature that i cannot understand?

I want seamless integration between every screen, and i want also every screen to be active even if i run the mail app in full screen on the internal monitor of the MBP.

Is this not possible? I can do this all the time on my Win10 machine, no problem. (other then it beeing a windows machine).

What i want is:
MacBookPro - Mail app running full screen mode all the time
External 2 monitors (one monitor with two cables) have seamless integration, so that windows that overlap the view of one "space" will be visible on the other one, as if it was one large screen.
(right now it hides the part that is on the other monitor surface)

Any solutions and i am very happy!

Thanks
//GF