[Samsung Galaxy Tab S3 Guides, News, & Discussion] klaberte:
Tested SM-T820, for Android O. It is reported that it works for Android P as well, with small modification (see below).

This post is designed to gather information found across this forum to give a step-by-step guide for obtaining a custom recovery TWRP and Root (via Magisk) for the Samsung S3 tablets running Android O. This process works if you are upgrading or staying with Oreo. [If you wish to run Android N, there is a slightly different process.] This process will wipe the contents of the tablet, so make backups as appropriate.

Credit to the software authors, original post creators (especially ashxy) and all those who contributed to the discussions to clarify all the issues. Most of the specifics below were hammered out in https://xdaforums.com/galaxy-tab-s3/development/recovery-twrp-3-1-0-1-samsung-galaxy-t3581359.

WARNING Flashing a custom recovery to your device will trip the Knox warranty bit on your phone. Check with your mobile provider to see if they will still cover warranty on Knox bit triggered devices. If you understand this risk, continue on. You generally don't have to worry, as it's incredibly hard to brick a Samsung device, and physical damage warranties should be exempt from Knox triggering. [credit: jcadduono]

The below instructions are for Android O (Oreo). User BenG7 reports (I have not verified) that the same process, with slight modification, will work for Android P (Pie). See posts #38 and #39 for those modified instructions.

1. DOWNLOAD

a. ODIN​

https://samsungodin.com/ (I used 3.13.1)​

b. Samsung USB Drivers​

https://developer.samsung.com/galaxy/others/android-usb-driver-for-windows​

c. Original firmware​

Search for SM-T820 and download latest version 8.X at http://updato.com/firmware-archive-select-model​

d. TWRP​

At some point, it will likely find an official version at https://twrp.me/Devices/. For now, grab the latest beta version at https://xdaforums.com/galaxy-tab-s3/development/recovery-twrp-3-1-0-1-samsung-galaxy-t3581359​

e. Magisk​

Get the latest stable Magisk version from https://xdaforums.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445​

f. Ashxy's oreo encryption disabler https://androidfilehost.com/?fid=890278863836288163​

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2. PREPERATION

a. While running your tablet in Android, go to Settings -> About device. Tap 7 times on Build number to enable Developer options. [credit: jcadduono]​

b. Now go to Settings -> Developer options. You need to enable OEM unlock, otherwise you might end up with boot problems or be unable to flash later on. Do not ever turn off Developer Options or OEM unlock. Doing so will cause your device to refuse to boot and require a full factory restore. [credit: jcadduono]​

c. Turn off your tablet.​

d. Get a micro SD card​

e. Copy Magisk and oreo encryption disabler onto microSD card​

f. Install microSD card into Tablet​

g. Install Samsung USB Drivers on Windows machine​

h. Unzip ODIN (1a) and Original firmware (1c) on your Windows computer​

i. While tablet is off, plug in usb cable between tablet and Windows computer​

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3. REFLASH ORIGINAL FIRMWARE (Optional if your tablet is already running Oreo)

a. Enter Download mode, press and hold Home, Down Vol, and Power until you see the Warning screen of the downloader mode. Press Vol up to continue​

b. Open up ODIN, confirm that ODIN can see your tablet. We will fill in three files for flashing. Click AP button on Odin. In popup, select AP_[blah blah].tar.md5 (whatever AP file you downloaded in step 1c). After selecting the file, Odin will check the signature of the file you downloaded and appear to freeze up. Let it finish, and it will report: <OSM> Checking MD5 finished Sucessfully.. <OSM> Leave CS.. In a similar way, fill in the BL and the CSC files. As these are much smaller, the signature checks will go much faster. [Use the CSC file, not the HOME_CSC file, in the CSC slot. This will completely erase your data on the tablet, which minimizes the likelihood of future problems. You will be erasing all of the /data partition later, so there is no point in saving your data now by using the HOME_CSC file.]​

c. Make sure that in the options, Auto Reboot is selected. Then click Start. The screen of the tablet should show a progress bar as your tablet is being restored to its original firmware.​

d. At this point, your tablet will reboot into the default recovery mode, with a full blue screen and an android figure with the spinning arrows. The tablet will then self reboot again (perhaps more than once). After a while, you will be greeted with the Welcome screen where you can select your language. Be patient.​

e. Power off the tablet using the power button​

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4. FLASH TWRP

a. Enter Download mode (press and hold Home, Down Vol, and Power) until you see the Warning screen of the downloader mode. Press Vol up to continue​

b. (Re) Open up ODIN, confirm that ODIN can see your tablet,​

c. Important We do not want the tablet to automatically reboot after installing TWRP. If the tablet does reboot into Android before completing the steps in Section 4 and 5, there is a good chance the stock recovery will overwrite TWRP. So click the Options tab on the left and deselect Auto Reboot. After doing that, click the Log tab so you can monitor progress​

d. Click AP button on Odin. In popup, select twrp-[blah blah].img.tar (downloaded in step 1d), but make sure that the BL and CSC slots are unchecked, i.e. we will only be flashing an AP file this time. Then click Start. You will see the progress bar on your tablet. Watch the log window of Odin and wait for the message "<OSM> All threads completed. (succeed 1 / failed 0)"​

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5. ENTER TWRP AND CONFIGURE

a. At this point, from the last section, you should still be in Download mode on the tablet. It is important that, after flashing TWRP, to go straight to TWRP without letting the Android System reboot. To do this, Hold [Volume Down] + [Home] + [Power] to get out of Download mode and immediately swap to [Volume Up] (while still holding [Home] + [Power]) when the screen blanks. [credit: jcadduono]​

b. In a few secs, you will see the TWRP first screen, you can let go of all the buttons.​

c. If you don't see then TWRP boot splash try again from the start of 4. FLASH TWRP.​

d. Swipe to Allow Modifications. (For some reason, TWRP has some issues with the touchscreen for some tablets. If you experience this, expect to make several swipes or taps until it properly registers. You have succeeded when you see the classic 8-button main screen of TWRP.)​

e. Click Wipe then Format Data (do not just Wipe data), type yes and let it complete. Then click back until you get to the 8-button main screen of TWRP.​

f. Click the Mount button. Select Storage button, and confirm that the Internal Storage shows non-zero MB. If it is, you know that the /data partition is now mounted properly. However, since we put the needed install files on the microSD, go ahead and select Micro SD Card as our storage. Click Back until you are back at the 8-button main screen.​

g. Click the Install button, install the Magisk zip file you downloaded in step 1e and copied to your microSD. Once finished, click Back to get back to the install screen (do not reboot yet).​

h. Install the oreo encryption disabler zip file from step 1f. When it completes, you can press the Reboot System button and let your tablet reboot into Android. You can also disconnect the usb cable connecting the tablet from the computer. The tablet may reboot itself once or twice, let it do its thing, and eventually you will see the Samsung Welcome! Screen, where you can select your language.​

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6. CONFIRM DATA ACCESS IN TWRP

a. Before going into configuring your tablet, go ahead and turn off the tablet and re-enter TWRP (Home, Vol up, Power).​

b. *Important* Since you will not be changing your system partition, this time, when you get to the "Keep System Read only?" screen, click the Keep Read Only button.​

c. In the 8-button screen of TWRP, click Mount, click Select Storage. Confirm that the Internal Storage is greater than zero MB. If it is, congratulations, you have TWRP installed with /data mounted.​

d. Reboot to System​

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7. FUTURE USE OF TWRP

a. Unless you are planning to make changes to the System, continue to choose the Read Only System when you return to TWRP.​

B. [NOT YET TESTED]If you need to make changes to System, e.g. flash a new ROM, go back to Step 5 and install the ROM just before Step 5g​

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8. Click the Thanks button if my guide helped you.