I recommend you to pick up 4u maths and TRY it.. see whether you get good assessment marks or not. 4u will make 3u seem a lot easier.
but it takes a lot of your time. drop japanese beginners, lowest scaling subject of yours, but if you truly enjoy it and are determined to do well, then of...
English Advanced - Look at sample essays and study guides.
General maths - just use the school textbook and focus more on past papers!
Economics - Riley, if you want basics down then get excel.
nahh shouldn't be an issue. there are people at my school that get 95+ with general maths, standard english, senior science and studies of religion. you can do it too! just do a bridging course during the long holiday.