
Samsung Galaxy Tab S3
The latest firmware for Samsung Galaxy Tab S3 (SM-T825N0) is T825N0KOU3CTD1 with Android 9 (Pie), released on April 2, 2020 for 1 region (CSC). Every official firmware build for SM-T825N0, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/T825N0KOU3CTD1
SM-T825N0 started on Android 8 and has received 1 major Android upgrade; it now runs Android 9 (Pie). The newest build, T825N0KOU3CTD1, was released on April 2, 2020. No new build has appeared for 79 months, so regular updates for this model have most likely ended.
Specifications of SM-T825N, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-T290N

SM-T537A

SM-T800X

SM-T807V

SM-T230X

SM-T230NY

SM-T237V

SM-T232
The newest firmware for SM-T825N0 runs Android 9 (Pie): build T825N0KOU3CTD1, released on April 2, 2020.
The latest build for SM-T825N0, T825N0KOU3CTD1, was released on April 2, 2020 (6 years ago).
SM-T825N0 has had no new firmware for 79 months, so regular updates have most likely ended.
Yes, all firmware files for SM-T825N0 are 100% official, original, and obtained directly from Samsung servers.
To flash SM-T825N0, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing SM-T825N0.