When doing the "heimdall flash --BOOT path/to/recovery-i9300.img --RECOVERY path/to/recovery-i9300.img"
I get the error message:
"libusb: error [_get_usbfs_fd] libusb couldn't open USB device /dev/bus/usb/002/063: Input/output error
ERROR: Failed to access device. libusb error: -1"
I suggest trying different USB ports on your laptop and trying different USB cables.
I tried one more time now with the intention of trying different USB ports and/or cables.
This time it worked at first try and I have no idea if I was using the same port and/or cable.
So I'll just assume that that was the problem and Kurtis' suggestion solved this problem :)