  A search for a narrow resonance with a mass between 350 and 700 GeV, and
decaying into a pair of jets, is performed using proton-proton collision events
containing at least three jets. The data sample corresponds to an integrated
luminosity of 18.3 fb$^{-1}$ recorded at $\sqrt{s} =$ 13 TeV with the CMS
detector. Data are collected with a technique known as "data scouting", in
which the events are reconstructed, selected, and recorded at a high rate in a
compact form by the high-level trigger. The three-jet final state provides
sensitivity to lower resonance masses than in previous searches using the data
scouting technique. The spectrum of the dijet invariant mass, calculated from
the two jets with the largest transverse momenta in the event, is used to
search for a resonance. No significant excess over a smoothly falling
background is found. Limits at 95% confidence level are set on the production
cross section of a narrow dijet resonance and compared with the cross section
of a vector dark matter mediator coupling to dark matter particles and quarks.
Translating to a model where the narrow vector resonance interacts only with
quarks, upper limits on this coupling range between 0.10 and 0.15, depending on
the resonance mass. These results represent the most stringent upper limits in
the mass range between 350 and 450 GeV obtained with a flavor-inclusive dijet
resonance search.
