Try to make pendulums oscillate by rhythmically pulling the rope. Notice: it should be done hastily. Only a certain right (different for every pendulum) frequency can make them swing with a large amplitude. Getting all three pendulums to swing with a large amplitude simultaneously is impossible. Note that the greater the pendulum length (the longer the ropes which the ball is suspended from), the longer the oscillation period is, meaning you need to pull the rope less often to set the pendulum swinging.
How does it work
Harmonic oscillations, natural frequencies, oscillation period, resonance, resonance frequency.
In 1940 a suspension bridge in Tacoma, Washington which crossed the Tacom Narrows strait collapsed due to the resonance. Its central span was almost 900 m (3,000 feet) long. The bridge vibrations were caused by strong winds.