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Would Biology Economics and Mathematics at Edexcel A levels be a viable combination for Law in the UK?
Yes thats fine. Any combination of academic A-levels is generally fine. The only issues may arise if you are taking 2 or 3 vocational/applied subjects (e.g. accounting, electronics) or subjects with overlapping content (e.g. business studies with economics). Multiple creative subjects may not be the best preparation either.
In your case you're doing three non overlapping traditionally academic subjects so no issues.