The Engineering courses at Cambridge normally last four years and lead to the degrees BA (with Honours) and MEng (Cambridge has no BSc or BEng degrees; all undergraduates in whatever subject take the BA).
At the end of the third year, you will qualify for a BA (with Honours). However, the course is designed with a four-year structure leading to the degrees of BA (with Honours) and MEng. This is the academic qualification which carries with it by one of the professional Engineering Institutions as fully satisfying the educational base for a Chartered Engineer (CEng). Students leaving after three years (which is extremely rare) would be leaving their professional education unfinished.