Samsung had the guts to release its powerful tablet, Galaxy Tab S3 with mediocre storage but what the fans even more furious was their decision to remove adoptable storage function from android. This means, you were not allowed to move applications and games stored on the internal storage of your Galaxy Tab S3 to micro SD card. Fortunately, an XDA developer user found a way to do.
For this, you will need to have Galaxy Tab S3 TWRP custom recovery installed on your device. Then, install adoptable_storage_enabler_ashyx and then this, no-verity-no-encrypt_ashyx. After this, reboot your tablet and look for a popup in notification bar that says "new SD card detected".
After that, click on “Set up” > “use as internal storage” > next > erase and format > Next and you’re done.
Optional – Move/Migrate existing data via "Settings —> Device maintenance —> Storage —> Storage Settings (three dots) —> SD card —> Migrate Data (three dots)".
After this, any application or game that you install will be installed automatically on the micro SD card.