From Cambridge website Law page;
Courses with a typical offer of A*AA generally require A1, A2, A2; and for courses with a typical A Level offer of A*A*A, offers are usually A1, A1, A2.
In some cases, two Advanced Highers and an additional Higher may be acceptable (eg when an applicant is prevented from studying more than two Advanced Highers due to reasons outside their control) – such applicants are considered on a case-by-case basis and should seek advice from the College to which they’re considering applying as early as possible.
If your school/college can only offer a limited range of Advanced Higher qualifications, or taking three Advanced Highers means travelling to different schools, please do indicate this on the additional questionnaire. so that this can be taken into account when assessing your application.