Samsung Galaxy S3 I9300 is not detected by fastboot under Ubuntu 14.04 When I connect my device in Download mode to my laptop, it is not detected by neither adb  nor fastboot commands:

fastboot devices and sudo fastboot devices returns nothing when device is connected in "Download mode".

fastboot device and sudo fastboot device returns &lt; waiting for device &gt; when 
device is connected in "Download mode" and nothing occurs if i disconnect and reconnect the device.

cat /etc/udev/rules.d/51-android.rules returns:

SUBSYSTEM=="usb", ATTR{idVendor}=="04e8", ATTR{idProduct}=="6860", MODE="0666"
SUBSYSTEM=="usb", ATTR{idVendor}=="04e8", ATTR{idProduct}=="685d", MODE="0666"


cat /lib/udev/rules.d/70-android-tools-adb.rules returns:

# Udev rules for letting the console user access adb.
# Vendor ids
# Important product ids
# d001 adb recovery
# d002 adb system

# Applies to 18d1
# 2d00 accessory
# 2d01 accessory,adb
# 2d02 audio_source (part of libgphoto2-2)
# 2d03 audio_source,adb
# 2d04 accessory,audio_source
# 2d05 accessory,audio_source,adb
# 4ee1 mtp (part of libmtp-common)
# 4ee2 mtp,adb (part of libmtp-common)
# 4ee3 rndis
# 4ee4 adb,rdnis
# 4ee5 ptp
# 4ee6 ptp,adb
# 4e41 mtp
# 4e42 mtp,adb
# 4e43 ptp
# 4e44 ptp,adb
# 4e23 rndis
# 4e24 rndis,adb

ACTION=="add|change", SUBSYSTEM=="usb", \
  ATTRS{idVendor}=="18d1", \
  ATTRS{idProduct}=="2d01|2d03|2d05|4ee2|4ee4|4ee6|4e42|4e44|4e24|d001|d002", \
  TAG+="uaccess"

# Applies to 04e8
# 685b mass_storage
# 685e mass_storage,adb
# 685c mtp (libgphoto2-2 libmtp-common)
# 6860 mtp,adb (part of libgphoto2-2 libmtp-common)
# 6862 rndis,dm
# 6863 rndis
# 6864 rndis,adb &amp; rndis,acm,dm
# 6865 ptp
# 6866 ptp,adb
# 6864 rndis,dm
# 6865 ptp
# 6866 ptp,adb

ACTION=="add|change", SUBSYSTEM=="usb", \
  ATTRS{idVendor}=="04e8", \
  ATTRS{idProduct}=="685e|6860|6864|6866|6866", \
  TAG+="uaccess"


cat /lib/udev/rules.d/70-android-tools-fastboot.rules returns:

# Udev rules for letting the console user access fastboot
# 4e40 grouper
# 4ee0 mako/manta
# 4e30 maguro/toro

ACTION=="add|change", SUBSYSTEM=="usb", \
  ATTRS{idVendor}=="18d1", ATTRS{idProduct}=="4e30|4e40|4ee0", \
  TAG+="uaccess"

# Applies to 04e8
# 686d odin mode (Download mode, Bootloader mode)    

ACTION=="add|change", SUBSYSTEM=="usb", \
  ATTRS{idVendor}=="04e8", \
  ATTRS{idProduct}=="685d", \
  TAG+="uaccess"


lsusb returns Bus 002 Device 015: ID 04e8:6860 Samsung Electronics Co., Ltd GT-I9100 Phone [Galaxy S II], GT-I9300 Phone [Galaxy S III], GT-P7500 [Galaxy Tab 10.1] if device is booted normally or in recovery mode.

lsusb returns Bus 002 Device 024: ID 04e8:685d Samsung Electronics Co., Ltd GT-I9100 Phone [Galaxy S II] (Download mode) if device is booted in "Download mode" (power-on device with Power Key + Home Key + Volume Down)

adb device returns 4dxxxxxxxxxxxxxx  device when device is booted normally.

adb device returns 4dxxxxxxxxxxxxxx  recovery when device is booted in recovery mode.

adb detects device and can reboot it in both normal mode and recovery mode by following commands: adb reboot and adb reboot recovery. But if run adb reboot bootloader or even sudo adb reboot bootloader just boots device in normal mode. To boot device in Download mode I have to manually press Power + Home + Volume Down keys simultaneously. 

I tried restarting my laptop and device, but no success.
My android-tools-fastboot and android-tools-adb and android-tools-adbd versions are: 4.2.2+git20130218.
Samsung devices with Download Mode do not support fastboot.

adb is not supported in fastboot mode anyways (nor Download mode).

If you want to flash something via fastboot, you must use Odin or Heimdall (or another tool that works with Download Mode).