My mac mini has 3 usb disks connected, seems not possible to change the placemet on the screen/desktop.

You should be able to drag the external drive icons into any order that you want. For them to keep their place and order, they need to be placed directly below the internal drive icon on the upper right hand side of the desktop. If there are other non drive Icons in that area, then you need to move those out of the way first.

Check whether you have your Finder preferences set to sort the desktop icons, or if they're unlocked.

Ctrl-click on your desktop and check the Sort By sub-menu. If it's anything other than 'None', then the Finder is managing the icon placement for you.

If they're managed, the Finder will list everything on the desktop based on the selection.

If unmanaged, you can drag them wherever you want, and the placement should be persistent.