It's up to you, all three are pretty much same in difficulty.
I take both accounting and legal, and personally I find accounting easier than legal. However, if you do not enjoy what it entails, don't take it. The amount of perceived liking of the subject correlates directly to marks, though, that's the same for every subject I guess? This is a generalization. Psychology, I do not do, but my friend does it and took legal last year. She says Psych is much easier than legal. Most of the people in her class seem to do well.
Legal, you don't really have to like the subject to do well. I dislike it, because its so rote-learning based compared to my other subjects. If you are good at memorization, do it. It also helped me improve note taking in other subjects. Scaling: -1
Accounting: Understanding concepts. 66% practical application, 33% theory Scaling: +1
Psych: ? apparently easier than legal Scaling: -2
It's up to you and your strengths whether you'd be good at those subjects...if you're good at english, you'll probs be good at legal+ psych. If your more maths based, logical, go for accounting