3. (5 points) Using the digits $1, 2, 3, 5, 6, 7, 8, 9$, the maximum number of prime numbers that can be formed (each digit can only be used once and must be used) is 6.
【Solution】Solution: The following prime numbers can be formed:
2, 3, 5, 7, 61, 89, a total of 6.
Answer: Using the digits $1, 2, 3, 5, 6, 7, 8, 9$, the maximum number of prime numbers that can be formed is 6.
Therefore, the answer is: 6.
3. (5 points) Using the digits $1, 2, 3, 5, 6, 7, 8, 9$, the maximum number of prime numbers that can be formed is $\qquad$ (each digit can only be used once, and must be used).