Make sure you have TWRP installed on your device to be able to flash the ROM. You can get it for the Galaxy A3 from here: Download
Download one of the builds above and copy them to a place where you can find it later. It doesn't matter if it's on the SD-Card or internal storage.
Optionally download GApps for using Google related services. Choose ARM as architecture and dependent on your downloaded ROM 8.1 or 9.0 as the OS version.
Reboot your phone into recovery mode with the combination VOL +, Power and Home. There you wipe the partitions Data, System, Cache and Dalvik.
Select and flash the zip files from the path you downloaded them. First the ROM and then the optional GApps. Reboot your phone now and you're done!