[Android General] MotoJunkie01:
This guide, for the Lenovo Tab 3 8" (TB3-850F), has 3 purposes:

1. Unlocking your bootloader;

2. Installing TWRP custom recovery;

3. Attaining superuser privileges (root) of the stock OS.

NOTE:

I can attest & confirm that this method has worked 100% on at least 7 devices of my own. Please read and re-read the instructions carefully before proceeding. If you follow this tutorial exactly as delineated, you should have no adverse issues in unlocking your bootloader, flashing custom recovery, or achieving systemless root of the stock Android OS.

MENTIONS & THANKS:

Based on XDA Senior Member [MENTION=6939319]Hemphiz[/MENTION]'s groundbreaking root and custom recovery for this device, I am now revising this thread wth improved step-by-step instructions based on members' feedback and experiences. Thanks [MENTION=6939319]Hemphiz[/MENTION] for your research, assimilation of necessary files, and for breaking ground on this tablet. Thanks also for your granting me ownership of the thread. Your contributions are greatly appreciated by many. Thanks to the great [MENTION=631273]Chainfire[/MENTION] for his flashable systemless rooting script. Thanks to [MENTION=6596524]jemmini[/MENTION] for compiling TWRP for this tablet. Finally, thanks to the many members who have tested this guide and provided much needed input & user experience.

DISCLAIMER: As always, please be reminded of the risk involved with flashing a custom recovery and/or attempting to attain superuser privileges of your Android OS. I will not be held responsible for devices bricked or otherwise rendered inoperable as a result of this thread. You assume full responsibility for any good or bad which may result from proceeding any further. However, with that having been said, I will provide any and all support possible. Read and follow the foregoing steps thoroughly and carefully prior to proceeding. NOTE: THIS GUIDE IS INTENDED ONLY FOR THE LENOVO TAB 3 8"/TB3-850F.

REQUIREMENTS:

1. Minimal ADB & Fastboot v1.4 (or newer) Windows Installer;

2. Windows PC or laptop (Mac may be used; however this guide is intended for Windows 7 and newer operating systems);

3. Official Lenovo USB Device Driver v1.1.21;

4. TWRP v3.0.1 for the TB3-850F;

5. ADB Driver Installer for Windows;

6. SuperSU v2.82-SR5 flashable zip.

(All files, drivers & utilities for this process are available in the below download link)

DOWNLOAD LINK: https://drive.google.com/folderview?id=1l-YN5aXmjDF3O1bHxFHVIxLKuvd847Q9

INSTRUCTIONS:

1. First and foremost, unlocking the bootloader is necessary. Also, this step is a good place to make some requisite preparations. Power on your Tab 3 and select SETTINGS once booted up. Go to ABOUT DEVICE, find BUILD NUMBER, and tap it 7 or 8 times until DEVELOPER OPTIONS is enabled. In DEVELOPER OPTIONS, tick OEM UNLOCKING and USB DEBUGGING. On your PC, install the Minimal ADB & Fastboot utilities, the ADB Driver Installer, and the Lenovo USB Driver Installer. Connect your Tab 3 to your PC with the supplied (or quality equivalent) OEM charging/syncing cable. Open a command window in your ADB/Fastboot directory, by holding SHIFT and right clicking in the desired path. In the command window, type: adb devices & press RETURN. If your device is connected properly, the command window will display a string of numbers/letters, which represent your device's serial number. If your serial number does not appear in the window, try changing USB ports, change syncing cables, or try reinstalling the ADB Drivers. Once properly connected, type adb reboot bootloader & press RETURN. Your tablet should boot into Fastboot Mode, which will be displayed in white bold letters on your device's display once complete. Once in Fastboot Mode, type fastboot oem unlock & press RETURN. Your device will prompt you to press Volume Up to proceed with unlocking the bootloader, or Volume Down to cancel the process. WARNING: UNLOCKING THE BOOTLOADER AUTOMATICALLY INITIATES A FACTORY DATA RESET. YOU WILL LOSE ANY UNSAVED VIDEOS, MUSIC, PERSONAL FILES, ETC.

2. Now that your bootloader is unlocked, you are ready to flash TWRP. Download the twrp.img file from my link and save it to your PC in the ADB/Fastboot path/directory. Also, save a copy of the file on your device's internal storage or external micro SD card. Since your device underwent a factory data reset, you will again need to enable DEVELOPER OPTIONS & tick OEM UNLOCKING & USB DEBUGGING as you did in Step 1. When you have done this, again open a command window, ensure a proper connecion, & type adb reboot bootloader & press RETURN. Once in Fastboot Mode, type fastboot boot twrp.img & RETURN. EXTREMELY IMPORTANT NOTE: this device's boot image is secured with dm-verity. Leave the /system partition as read-only when prompted by TWRP. Mounting /system as read/write at this point will result in a boot loop. Flashing SuperSU, as outlined in Step 3, will patch dm-verity and fix this.

From the TWRP main menu, select INSTALL, select the image file option in the bottom right of the screen, navigate to the location where you saved twrp.img in device storage or external storage, and commence the installation. Be certain and correctly choose "recovery" when prompted by TWRP. Now select REBOOT, then RECOVERY. Your TWRP custom recovery should now be successfully installed on your device's /recovery partition.

3. Now, it is time to root this tablet. Download & save a copy of the SuperSU v2.82-SR5 zip file to the tablet's internal or external storage. In TWRP main menu, select INSTALL & navigate to the saved SuperSU zip. Commence with installation. Systemless root mode will unpack, modify, repack and flash the patched boot image. Once done, reboot system. Your Android OS should now have full systemless root, and the /system partition may now safely mount as read/write.

TROUBLESHOOTING:

In the event your device becomes stuck in a bootloop, or is otherwise failing to boot into the OS, the following is a link to my stock firmware images thread for this device. NOTE: the following link will be beneficial only if your device successfully boots into Recovery/Fastboot mode. This link provides stock partition images of /boot, /recovery, /logo & /system. Typically, flashing the stock boot & system images will get you up & running again, and will restore the Android OS to stock condition.

https://xdaforums.com/android/general/rom-lenovo-tab-3-8-tb3-850f-android-6-0-t3593043/page5

In the unfortunate event that you encounter a brick (your device fails to boot into the OS or Recovery Mode), please visit [MENTION=8243079]r&xp[/MENTION]'s thread on unbricking your device & restoring it to factory stock condition. Members using his thread be sure to thank [MENTION=8243079]r&xp[/MENTION]. His collection of necessary files & comprehensive instructions has saved many, many bricked tablets. https://xdaforums.com/android/help/lenovo-tab-3-8-tb3-850f-unbrick-root-t3598727