Depends on the reasons for taking three years and which universities you apply to: I'm sure a lot of universities will give you an offer if you have taken A levels over 3 years (even if their official policy says they don't). However, unless you have good reason (e.g. ill, etc.) then taking A levels over three years is generally not accepted by the main Russell Group law schools (Bristol, UCL, Oxbridge, LSE, KCL, etc.)