OpenFL 3 don’t have native extension capability yet (Joshua is rethinking it to make it integrate better with SDL2), you better off stick with legacy OpenFL if you still need it. Or maybe try Hybrid mode (?).
EDIT: After thinking about it more, I guess you could always go through cpp and call jni from there (assuming you update/create your extension). AFAIK so far only JNI is being deprecated (for now?) but I think you still can use cpp route.