= Solution
With zero initial velocities, choosing the initial displacement proportional to one <eigenvector> excites only its <normal mode>. Suitable ratios are
$$
\boxed{x_1:x_2:x_3=1:1:1,\qquad1:0:-1,\qquad1:-2:1.}
$$
The overall scale must remain small compared with $l$. Release each configuration from rest; its subsequent vector is its initial displacement multiplied by $\cos\omega_jt$. The middle bob is stationary in the second mode, while it moves oppositely with twice either outer amplitude in the third.
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