You have 3 weeks, still a ton of time left before you can change so don't stress. Stress will destroy you in A Level, so always be calm and organised.
First of all, you should look into universities as well as their courses and see what you would like doing. You want to create a lot of options for yourself when it comes to university.
Secondly you should look into the specification of each subject and see what you would enjoy doing, but remember no A Level is a walk in a park, you will have times where you think wtf am I doing in this class and in other times you'll have the time of your life breezing through the content, this is normal.
Teachers always teach at a speed in which the whole class can cope, if you do your own work outside class you can get through the whole two years of content briefly in the first year and you can use the second year solidifying your knowledge and exam technique.
However the most important thing to do is to look at university courses and see what your future will look like, it ultimately depends on what you enjoy doing as well as your goal in life.
Any other questions don't hesitate, I've been through it all!!