Ok, back to my problem. I found out, that the order of the windows changes when ever I press the "show desktop"-icon in my task-bar, which I do quite often.
With every click on that icon the order changes. (with chosen "on the desktop"-position in the settings). But that appears to be much clicking, as all other windows I use (Firefox, Word, etc.) appear and disappear with clicking the icon. The only possibility then would be to minimize all windows by hand and then when I see the desktop I could click as often on the "show-desktop"-icon as I wanted to get the right order of the three calendar-windows. But that's not really practical.
The same whith the option "in the backround". Here the calendar-windows disappear when I click on the desktop-icon. And again I only can see the calendar if I minimize all other windows each by hand and then click the icon.
I hope you guys understand what I'm trying to say.