From my experience, 3U maths just takes way too much effort and time. It does scale really well though. Honestly it's hard to judge with just your raw marks. If you think the scaling would help you then you should keep it. But if the work load gets too big I would drop it. If you score 90s in low scaling subjects I think it's still better than failing in a high scaling subject. If the work load isn't stressing you out you should just keep them all.