For me, 3 pieces are not enough. Basically I did about 20 pieces a year. For example If I plan to take grad 6, I would do 9 pieces at grade 6 level, 7 pieces at the lower grade levels, and 4 at grad 7 level.
Since I did 9 piece in grade 6 level, I picked only 3 that I could perform best. Usually my piano teacher assign me harder and easier pieces alternatly. However, we didn't depend on the grade syllabus that much. I learned what I want to learned.