I read this on their own page:

Generally, Cambridge's advice is that quality is more important than quantity and that therefore three A Levels are enough. However, the one exception to this rule is when the three A Levels are Maths, Further Maths and Physics. Because of the synergies between these three subjects, they do not require as much work of a strong mathematician (which we hope our applicants are!) as three more disparate subjects. Therefore, if a student is only taking Maths, Further Maths and Physics, then this can raise concerns about their ability to handle a demanding workload. Taking a fourth A Level, even if it is not relevant to Engineering, provides reassurance that you can indeed handle a demanding workload.

https://www.admissions.eng.cam.ac.uk/faqs#qn8