I always hold the device in my hand and scroll the screen with my thumb. I found it uncomfortable on my wrist to have to use two fingers to press twice to undo it, and it sometimes didn't flow as it should, and I had trouble using three fingers to press the redo because it became undo, which made it very frustrating. I believe there should be a setting for how many fingers you want to use to undo and redo.
The picture is the way I hold my device.