
Samsung Galaxy A3
Every official firmware build for SM-A300X, newest first: one row per build with all regions (CSC) that received it.
Specifications of SM-A300, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-A300XU

SM-A307GT

SM-A305YN

SM-A310X

SM-A405S
You can find the firmware on the model: SM-A300X page.
Yes, all firmware files for SM-A300X are 100% official, original, and obtained directly from Samsung servers.
To flash SM-A300X, 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-A300X.