
Samsung Z3
The latest firmware for Samsung Z3 (SM-Z300H) is Z300HDDU0BPI2 with Android 2.4.0.1, released on October 10, 2016 for 1 region (CSC). Every official firmware build for SM-Z300H, 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/Z300HDDU0BPI2
All 10 builds in our database run Android 2.4.0.1. The newest build, Z300HDDU0BPI2, was released on October 10, 2016. No new build has appeared for 122 months, so regular updates for this model have most likely ended.

SM-Z200M

SM-Z400F

SM-Z130H
The newest firmware for SM-Z300H runs Android 2.4.0.1: build Z300HDDU0BPI2, released on October 10, 2016.
The latest build for SM-Z300H, Z300HDDU0BPI2, was released on October 10, 2016 (10 years ago).
SM-Z300H has had no new firmware for 122 months, so regular updates have most likely ended.
Yes, all firmware files for SM-Z300H are 100% official, original, and obtained directly from Samsung servers.
To flash SM-Z300H, 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-Z300H.