Given the equation: \[ \sin \left( \frac{A}{2} \right) \cdot \sin \left( \frac{C}{2} \right) = \sin \left( \frac{B}{2} \right) \], where \( 2s \) is the triangle's perimeter. Applying the sine rule and relationships between sides and angles, along with the semi-perimeter formula, and solving based on the given conditions, yields the result: \[ s = 3b \]. Therefore, the correct answer is \( 3b \).