In an equilateral triangle, the sum of the perpendiculars drawn from any point inside the triangle to the sides of the triangle is constant. Using this property, we can find the side length using the formula provided.
\[
\text{Side length} = \frac{2}{\sqrt{3}} \left( a_1 + a_2 + a_3 \right)
\]
Final Answer: The correct answer is (a) \( \frac{2}{\sqrt{3}} (a_1 + a_2 + a_3) \).