How to make it possible to save raw photos on Samsung Galaxy A3 2017?
Camera2 API alone doesn't necessarily mean you can shoot RAW, though you can run the free app Manual Camera Compatibility. If it shows that your phone supports RAW, you should be able to shoot RAW with the Manual Camera app itself. 

Photoshop Lightroom is another app capable of capturing and editing RAW, so maybe that's worth a shot aswell.