Do whatever subjects you think you will get the best grades in.
I can assure you that the whole "do subjects x, y, z to get into law" rhetoric that is spread among people who don't even have plans of reading law at university is often false. Very few courses expect you do particular subjects. Law is not one of them.
However, if you do have an interest towards commercial law, I would recommend you consider an A-level in economics. It is a very interesting subject and should develop your commercial awareness a lot. But that is purely just a suggestion.
If you are happy with your three choices as of now, go ahead with them. A-levels can be tough when you study subjects you are not particularly inclined towards so I would say you are on the right path by selecting those three subjects that you feel confident/interested about.