The problem is interesting, but being vague, numerous solutions seem possible:

a1: 2 is the smallest prime

a2: 2 is the only **even** prime.

a3: 2 is the only even number equal to its factorial.

b1: this is the only triplet of a square, a prime and a triangular number in a consecutive sequence

b2: this is the only triplet of digits in sequence in which the middle number's factorial is equal to the product of all the members

b3: this is the only triplet of a square, a prime and a perfect number in a consecutive sequence.

Need I CONTINUE?

Let's wait and compare with the official solution

??? How is it possible to edit a typo in my comment's title?

*Edited on ***September 2, 2014, 8:18 pm**