@kpt-fish, for i9300 Heimdall 1.4.0 works fine as Odin do.
If you have Windows, for your first flash, use Odin.
Be sure Samsung’s driver are well installed, boot your phone in Download Mode, connect it to your computer and flash TWRP 3.2.3.0 (here https://dl.twrp.me/i9300/)
Reboot in recovery mode, with an sdcard containing /e/ zip file and flash it.
You should be done !!