You don’t have to root your device to flash a rom or a GSI. You just need to have a custom recovery like TWRP. TWRP has to be flashed using Odin. Then in TWRP you can make a backup of your official rom (Save it on your SD card, not on the internal memory of the phone. By default it’s on the internal memory, so change it because maybe you will wipe it by accident, and make a copy on your computer). But you also can download an official rom on websites like sammobile (.com) (you can trust this site) and then (re)flash the official rom you’ve downloaded, using Odin, if anything goes wrong.
I’ve said “do a back up of your EFS partition” because this is always the first thing to do just after you’ve flashed a custom recovery. Sometimes, losing EFS partition can happen, and then your phone won’t connect to network. I didn’t have this issue with my A320FL although I flashed it several times with different roms and GSI, but I had this problem with several S4 Mini (i9195), so now I do a back up immediately after flashing a custom recovery before doing anything on the device.
As a beginner, myself, I suggest you what I’ve done : buy a second A3 2017 in poor condition (with front glass broken for example, but touch screen ok, and be sure that the usb port is not faulty/loosy, because it’s important when flashing from your computer), it doesn’t cost a lot, and then make your experimentations on it. When you will be successful, you can do the same on your daily device. Yes, e0s on A3 2017 seems to be good for daily use. Just be aware that you will lose quality with your camera with every roms which are not official (I mean every rom which is not a Samsung official rom or your Telephony Provider rom). I don’t do a lot of things with mine (just calls, sms/mms, photos, listening podcasts, I tried internet with a web browser and it works). My battery lasts 12 days and listening podcast 1 hour everyday takes 30% of the battery, so the autonomy is very good.
Concerning your “getprop ro.treble.enabled = false”, I don’t understand what you mean exactly by that. You have to flash a repartitioner script (with TWRP or OrangeFox) to treblize you device, before flashing a GSI. The ro.treble.enabled options is in the build.prop file which is in the system partition of a rom, it will be “= true” in every treble rom you will install. The official images of eOs GSI are now in a zip, (there is also an .xz format, that’s why aibd talked about .xz file). But, it’s not a flashable zip, you have to extract the .img file, but before you have to do some things to prepare your phone.
As I am not an expert at all, I don’t know if it’s possible to keep the original vendor files of the original official rom of an A3 2017 and treblize the phone and just flash the GSI of eOs and have it working like that.
There is LeOS (a GSI of an ungoogled LineageOs which is developed by harvey186 and with his GSI he recommends an OEM vendor. But I don’t know if it’s possible with a no real treble device like A3 2017 which is artificially trebled).
If it’s not possible to keep your official vendor, I suggest you first to flash a treble custom rom found on the forum of xda-developers, follow the tutorials you can find on the pages where are located the different custom roms to learn to treblize your device, etc… but there are some things to know : you also have to flash other things. I can write what i’ve done (I’ve first flashed a treble custom rom found on xda-developers, before I found an eOs for A3 2017). You seem to already have TWRP because you say to try to flash zip, but here is what I’ve done to flash TWRP and then a treble custom rom :
Back up photos and other things on a computer.
Activate Developer Options on the phone
activate OEM unlock
enable USB debogging enabled
restart in download mode : volume down + home + power
Open Odin on the computer.
load the twrp file in the AP area of Odin
Untick auto reboot ! That’s important.
plug the phone
click start
when it’s done : don’t restart the phone ! but restart in recovery mode by doing this : volume down + home + power and when the screen is black release quickly the buttons and then repush immediatly these 3 buttons : volume UP + home + power, this will open the recovery mode (in TWRP)
Now in TWRP :
back up the EFS partition
format data
flash noverity script (found on xda-developers)
flash rmm bypass by corsicanu (found on xda-developers)
back up the official rom if wanted (on the SD card and be aware that it will take time and space)
restart TWRP (available in menu)
format data and then wipe everything except external (sd card) and usb otg
To treblize the phone : flash a repartitioner script (found on xda-developers)
reboot TWRP again
format data and wipe everything except sd card and usb otg
flash a treble rom (crDroid in my case, found on xda-developers).
when flashing is done, wipe cache/dalvik as suggested by TWRP
reboot the phone
see if the phone boots
if it’s ok, reboot in recovery mode (TWRP), try to wipe data and if there is an error, then format data and then rewipe data. I’ve got this error, and if you too, it will persist everytime you will go in recovery mode and won’t allow you to make a back up of your data, so I personnaly use OrangeFox (which is flashable with TWRP). But OrangeFox doesn’t permit to flash a .img on specific partition (maybe I’m wrong) whereas TWRP do. So, to flash an .img on the system partition, you will have to use TWRP.
But, as I said in the post mentionned by aibd, I don’t use a conventional way to flash the official GSI of eOs.
This is what I’ve done, I’m not responsible if you brick your phone, this is just what I’ve done, not what I’m telling you to do ! But I’m telling you to make your experimentations on a second phone !
And, please, read a lot of topics about flashing treble roms on A3 2017 on the forum of xda-developers, here you will find lot of informations :
Be careful to use what is aimed to be used with A320, not a different phone. All the A series phones are in the same section of the forum.
Please someone correct me if I’ve said something wrong. This was just a beginner post.
