Not sure if this has already been reported or is of any use to anyone but I have today had my S970 supporting three USB drives at the same time, concurrently. So for instance in the save reg file page I can see USB1, USB2 and USB3 at the same time and the kybd seemed able to save and load from each.
It is a slightly odd setup so I will explain.
I have an oak cabinet I built for the keyboard, and it has three USB sockets at the front for easy access, one is for connecting the kybd to a PC if needed so is not really relevant here, the other two plug into a USB hub behind the kybd which plugs into the kybd USB, the usual sort of setup that I know many people use.
Today I had my usual USB pen drive plugged in but also had my panasonic RR-XS400 mini voice recorder plugged into the other USB socket to charge. The recorder is also a USB device in its own right so appears as a second USB device, nothing new there. But I noticed that when I was on the USB page on the kybd that it showed three USB selections (USB1, USB2 and USB3). Odd, so I scrolled to USB 3 and it showed nothing but a P1 entry and all options blanked out. Then I remembered that the recorder has an SD card slot for extra storage. So I put a spare SD card in and the kybd then saw it fine and was allowing me to load and save from the third USB, USB1 being my normal pen drive, USB 2 being the internal memory on the recorder and USB3 being the SD card on the recorder.
So in itself not tremendously useful but it proves that the S970 kybd WILL address more than two USB devices at the same time. So if you could insert a device that had a second and third pen drive addressed in the same way as the Panasonic recorder shows its internal and external memory, then three USBs should work fine I suspect.
Hope it may be of use to someone.