AC3 Codec for MX Amlogic MX2 Android Media Player Box
AC3 is not by any means the most common file type/format for audio files.
MP3 and Apple's AAC/m4a is do l far more often used especially on mobile devices and on the internet.

Downloading codecs like on PC is not possible. Android has to ship it in it's system or your media player has to support AC3 files.
This is also not very easy because AC3 is patented and licensed so developers have to use free codecs.