When it comes to flashing your device I would suggest going to the XDA forums. You can usually find all kinds stuff such as Roms(stock or custom) for the devices as well as instructions on how to flash them, toolkits that help simplify flashing, custom recoveries and other useful information. I do not know what version of the A3 tab you own, but I will supply a link to XDA website that may or may not help.
http://forum.xda-developers.com/android/general/acer-ax-b1-t2875894
http://forum.xda-developers.com/showthread.php?t=2240029
Good luck.