= Solution
A <normal mode> has a fixed pattern of relative <displacements>, with every coordinate proportional to one common time-dependent factor. There are \b[three independent normal modes] in this three-coordinate model: two internal vibrations and one zero-frequency <rigid-body translation>. The latter moves all three atoms together without stretching either spring. This count concerns the specified one-dimensional model, rather than a molecule with unrestricted three-dimensional motion.
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