= Solution
If an exterior planetesimal reaches conjunction just before apoapsis, its radius is increasing. The closer pre-conjunction pull from the trailing inner planet removes more <specific angular momentum> than the more distant post-conjunction pull restores. Its semi-major axis falls, its <mean motion> rises, and the next conjunction moves later in its orbit toward apoapsis. A conjunction just after apoapsis produces the reverse imbalance: the stronger post-conjunction pull adds angular momentum, raises the semi-major axis, and shifts the next conjunction earlier. The conjunction phase is therefore restored toward apoapsis.
The symmetric configuration has one of the three conjunction branches at apoapsis and the other two symmetrically placed. It has
$$
\boxed{\phi_{1,\rm eq}=q\pi=3\pi\equiv\pi\pmod{2\pi}},
$$
so the <resonant argument> librates about $180^\circ$.
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