I got the eOS image from https://community.e.foundation/t/unofficial-build-samsung-galaxy-s-iii-i9300-i9305-for-e-os-pie/60800 and the TWRP image from https://twrp.me/samsung/samsunggalaxys3internationalexynos.html

The image from twrp.me is named twrp-3.7.0_9-0-i9300.img but I had to rename it in twrp-3.7.0_9-0-m0.img to make OpenAndroidInstaller happy.

$ adb devices -l
List of devices attached
xxxxxxxxx device usb:1-2 product:m0xx model:GT_I9300 device:m0 transport_id:2