
Samsung Galaxy Trend3
The latest firmware for Samsung Galaxy Trend3 (SM-G3509) is G3509KEUAMK1 with Jelly(Android Bean), released on November 5, 2013 for 1 region (CSC). Every official firmware build for SM-G3509, 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/G3509KEUAMK1
Specifications of SM-G350, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-G3502L

SM-G3589W

SM-G357M

SM-G3568V

SM-G355M
You can find the firmware on the model: SM-G3509 page.
Yes, all firmware files for SM-G3509 are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G3509, 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-G3509.