The greatest common factor of A and B is 15, and the greatest common factor of B and C is 9. What is the greatest common factor of the three numbers A, B, and C?
甲数和乙数的最大公因数是15，乙数和丙数的最大公因数是9．甲、乙、丙三个数的最大公因数是多少？
<result>3</result>