How many hexadecimal strings of length twelve have eight digits from the set 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and four digits from the set A, B, C, D, E, F? No repetition is allowed. How many hexadecimal strings of length twelve have at most three distinct digits? For this question only - evaluate your final expression and provide the number too.