Using a custom bootloader for my smartphone: TWRP on Samsung Galaxy A3

I read about /e/ some days ago and i decided to install this on my old smartphone (a samsung A3).

So first: What is /e/?
https://doc.e.foundation/

So i started at

https://doc.e.foundation/devices/a3xeltexx/install
where i learned, that i have to install a custom bootloader on my smartphone. This is very good explained at /e/:

(You will find there the description how to install "heimdall" and "adb")

schroff@zerberus:~/bin$ sudo ./heimdall flash --verbose --RECOVERY ~/Downloads/twrp-3.3.0-0-a3xelte.img --no-reboot
Heimdall v1.4.2

Copyright (c) 2010-2017 Benjamin Dobell, Glass Echidna
http://www.glassechidna.com.au/

This software is provided free of charge. Copying and redistribution is
encouraged.

If you appreciate this software and you would like to support future
development please consider donating:
http://www.glassechidna.com.au/donate/

Initialising connection...
Detecting device...
      Manufacturer: "SAMSUNG"
           Product: "Gadget Serial"

            length: 18
      device class: 2
               S/N: 0
           VID:PID: 04E8:685D
         bcdDevice: 021B
   iMan:iProd:iSer: 1:2:0
          nb confs: 1

interface[0].altsetting[0]: num endpoints = 1
   Class.SubClass.Protocol: 02.02.01
       endpoint[0].address: 83
           max packet size: 0010
          polling interval: 09

interface[1].altsetting[0]: num endpoints = 2
   Class.SubClass.Protocol: 0A.00.00
       endpoint[0].address: 81
           max packet size: 0200
          polling interval: 00
       endpoint[1].address: 02
           max packet size: 0200
          polling interval: 00
Claiming interface...
Setting up interface...

Initialising protocol...
Protocol initialisation successful.

Beginning session...

Some devices may take up to 2 minutes to respond.
Please be patient!

Session begun.

Downloading device's PIT file...
WARNING: Empty bulk transfer after receiving packet failed. Continuing anyway...
PIT file download successful.

Uploading RECOVERY
0%ERROR: Failed to unpack received packet.

ERROR: Failed to receive file part response!
3%
7%
10%
14%
17%
21%
25%
28%
32%
35%
39%
42%
46%
50%
53%
57%
60%
64%
68%
71%
75%
78%
82%
85%
89%
93%
96%
100%
RECOVERY upload successful

Ending session...
Releasing device interface...

and then:

For more information take a look at https://twrp.me/