I have been having trouble pressing the side button two times when I want to turn voice over on/off. For that reason I've tried to create a custom gesture in accessibility/voice over/commands/all commands/shortcuts/turn voiceover on_off
I pressed "add gesture" and selected "press three times on the back of your iphone".
Testing it out - with voice over off - nothing happens. Same with another gesture - "press three times with two fingers. When I selected the gestures ios didn't give me any warning that the gestures were taken.
All the best,