For the business, are you better at exams or more coursework? That'll probably be your answer. Although I will say that many people say that btec is a LOT less stressful than an a level.
For your third subject, I took computer science and my advice would be that it's a subject that you REALLY have to be interested in to take. I had a mild interest in it, but my love for it was pretty much destroyed by the end of the year... Art could be good, but I know it's a lot of work. I think psychology would probably be the easiest of the three to take since most people who I knew took it said it was their easiest a level. At the end of the day though, it's all about which one you'd be willing to put the most time in since you're gonna be studying it hard for two years.
tl:dr maybe do some summer work on those three subjects and see which you enjoy most lol