On Ubuntu 15.10 I also have three monitors, one of which (the primary monitor) is centred above the other two. I'm using Intel® HD Graphics 4600 graphics processor.
My desktop icons are being placed in the "dead" space to the left of the topmost monitor, as you guessed.
On Ubuntu 15.10 I also have three monitors, one of which (the primary monitor) is centred above the other two. I'm using Intel® HD Graphics 4600 graphics processor.
My desktop icons are being placed in the "dead" space to the left of the topmost monitor, as you guessed.