How to reboot an Acer B3-A20 into safe mode? When holding down the Power off button on the screen, nothing happens till I let go, and then the system just powers off.
This guide is not exactly for your model, but it will probably work on your model, too:


  Turn on safe mode
  
  
  Turn device off.
  Press power button.
  When the "Acer" logo is displayed, release the power button.
  Immediately after releasing the power buton, hold the volume down button.
  Keep holding down the volume down button until the phone finishes rebooting.
  Release the volume down button when you see Safe Mode appear in the lower left corner of the screen.
  


Source: http://howtosafemode.com/843/how-to-turn-on-safe-mode-on-acer-iconia-tab-7-a1-713?show=845#a845