There are three numbers A, B, and C. 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 A, B, and C?
A、B、C三个数，A和B的最大公因数是15，B和C的最大公因数是9，A、B、C的最大公因数是多少？
<result>3</result>