It would be smarter to build Android Oreo Go,unfortunately no I do not have the experience to do it but it is just a suggestion.
i think that someone is developing lineage 15.1 for our beloved device
Looks like it's coming along nicely with a lineage-15.1 branch https://github.com/fourkbomb/android_device_samsung_midas
Here's an update on the development: https://blog.forkwhiletrue.me/posts/an-almost-fully-libre-galaxy-s3/ It's not for Oreo development specifically but it's about u-boot and the kernel.
the device tree for s3 on los15.1 already prepeared here.but midas still not perfect which needs a lot skills to run.so no body build a rom to try…(excepet the author)
https://github.com/fourkbomb/android_device_samsung_midas
Try it yourself
If you're after something to tinker with, have a spare GT-I9300/GT-I9305/GT-N7100/GT-N7105 lying around, and don't really care about using anything based on the old kernel (be that LineageOS, Replicant, or the stock firmware), you can even try this for yourself. While it should be possible to go back to the proprietary bootloader, I have not tested doing so: doing this would be left largely as an exercise to the reader. It's also worth mentioning that LineageOS/Replicant will boot under u-boot with a kernel compiled without CONFIG_ARM_TRUSTZONE set, but the display will not work (probably because u-boot does not initialise the display).
...
here you go- https://xdaforums.com/galaxy-s3/development/rom-lineageos-15-1-t3862953
It would be smarter to build Android Oreo Go,unfortunately no I do not have the experience to do it but it is just a suggestion.