Re: Lumia 520 Died during downgrade to 8.1 from 10.
i had the exactly same problem, you just need do keep trying, use "Lumia recovery tool 5.0.7"
the steps:
1 - open Lumia recovery tool
2 - click on the "My phone doesnt respond" button
3 - connect the phone when he ask you to
4 - wait and he will recognize your phone
try do restore, as you said ("But same thing happened after 3% it again failed"), it always give an error, BUUUUUT, everytime you try, the error is later at the progress, you will eventually get to 50%, when this happens, you will not get any error again
*everytime you try again, you will have to remove the battery and do all the process again (i know, its exhaustive, took me some hours to complete the process)
* you HAVE to use Lumia recovery tool, doesnt work with other programs
*remove the battery before connecting the phone at step 3, and wait a little after you click the button, something like 5 seconds