Just stick with Maths, Art and Economics.
Economics might be relatively similar to maths (though I haven't done either at A Level), so perhaps swap it for religious studies, media, or psychology. Media studies will be more artistic so great for your career choice; otherwise I'd go with psychology.
3 A Levels is enough, especially for a fashion designer, it's not as if you want to be a doctor. 3 will get you into university perfectly fine.