Observations and results
Did you find that the center of mass of the original rectangular structure was just below the middle marshmallow? When you added two more marshmallows to the middle one, did you find that the center of mass shifted up?
An object will more stably balance if a vertical line from its center of mass passes through the balance point. In this activity, the balance point was the spot you marked with an X, where you finger was, and the vertical line is the same as the one you drew. (This vertical line is also known as the plumb line.) By finding where the vertical lines intersected, you could figure out where the structure's center of mass was. You should have found that the first rectangular structure had a center of mass below the middle marshmallow, whereas the second rectangular structure's center of mass was higher up, within the middle marshmallow. Adding two more marshmallows on top of the middle marshmallow should have shifted the center of mass even higher up. Additionally, an object should be more stably balanced if its center of mass is under the balance point, but unstable if the center of mass is above the balance point. Consequently, the first structure you made (with just three marshmallows) should have been less stable than the rectangular structures and, likewise, the original rectangular structure should have been more stable than the one with two marshmallows added.
More to explore
Center of Mass , from Wikia: High School Online Collaborative Writing
Resource Lesson: Center of Mass , from PhysicsLAB Online
Center of Mass , from HyperPhysics, Georgia State University
Circus-Trick Science: How to Balance Anything , from Science Buddies