When you plug in external devices, there is the consideration of the sequential order in which they are initially attached. For instance, if I have two USB drives where one appears as G and the other H, if I later reboot and install the one that previously showed as H, it may show as G.
Also, if I have 3 USB drives and always use them one at a time, they may each appear as "F" every time I use them.
However, I did an experiment where I had the Disk Management dialog (under Computer Management-->Storage) open when I attached two USB drives. In the interface, I manually selected two different drive letters for those USB drives. One mapped as "H" and the other as "I" (by default they were "F" and "G"). I ejected the drives, then plugged in the one I had designated as "I". Sure enough, it showed up as "I", not "F" as I would have expected since "F" would be the next logical drive letter.
I suggest plugging in all three of your devices, then explicitly selecting drive letters for them in the Disk Management. Hopefully by doing this, the computer will remember each device with the previously designated drive letter and avoid any unexpected reassignments.