Flash/unbrick or upgrade your device with the firmwares below using odin
Learn how to use odin from here
Samsung Galaxy A3 SM-A300FU Stock Firmware
A300FUXXU1ANL4_BTU1ANL4 - Android 4.4.4
A300FUXXU1BOEC_BTU1BOE1 - Android 5.0.2
A300FUXXU1BOL3_VFR1BOL1_VOD - Vodafone - Android 5.0.2
Germany
A300FUXXU1ANL4_DBT1ANL2 - Android 4.4.4
Romania
A300FUXXU1ANL4_OXX1ANL3_ROM - Android 4.4.4
Italy
A300FUXXU1AOB3_ITV1AOB3 - Android 4.4.4
A300FUXXU1BOH5_ITV1BOH5 - Android 5.0.2
A300FUXXU1BOEC_ITV1BOEC - Android 5.0.2
Poland
A300FUXXU1AOC1_OXX1AOC1_XEO - Android 4.4.4
A300FUXXU1BOJ5_OXX1BOK1_XEO - Android 5.0.2
Germany
A300FUXXU1BOEC_DBT1BOE1 - Android 5.0.2
A300FUXXU1AOD3_VFR1AOD3_VD2 - Vodafone - Android 4.4.4
A300FUXXU1BOF7_VFR1BOF7_VD2 - Vodafone - Android 5.0.2
France
A300FUXXU1BOEC_XEF1BOE1 - Android 5.0.2
A300FUXXU1BOEC_SFR1BOF1 - Android 5.0.2
Spain
A300FUXXU1BOEC_PHE1BOF1 - Android 5.0.2
Greece
A300FUXXU1BOEC_EUR1BOF1 - Android 5.0.2
Nordic
A300FUXXU1BOEC_NEE1BOE1 - Android 5.0.2
For other Samsung firmwares, click here