Let's try an easy calculation: what's the moment of inertia of these three balls? Each ball has mass m = 3 kg, and they are arranged in an equilateral triangle with sides of length L = 10 m. We spin the triangle around the spot marked "X", which is one of the balls.
Okay, let's try a harder one. Suppose that we locate the center of mass of the triangle -- I've marked it with the "X" in the figure below. What's the moment of inertia of the triangle spun around this point?