Identify the Mandatory Prime Factor in the Three-Digit Number 3?0

To solve this problem, let's identify the prime factor that is certain to be a part of a number in the format $3?0$. This number always ends with 0, indicating it is divisible by 10.

Step-by-step Solution:

• The number ends in 0: $3?0$. This means it is divisible by 10.
• Since a number divisible by 10 contains the prime factors of 10, which are 2 and 5, it means both 2 and 5 are factors of any such number.
• Out of the given prime factor options, 3, 5, 11, and 7, the prime factor 5 is certain to be present because every such number must end in 0, confirming divisibility by 5.

Thus, the prime factor that will surely appear among the first factors of a three-digit number in the format $3?0$ is $\mathbf{5}$.