  We analyze the distribution of known multi-planet systems ($N \geq 3$) in the
plane of mean-motion ratios, and compare it with the resonance web generated by
two-planet mean-motion resonances (2P-MMR) and pure 3-planet commensurabilities
(pure 3P-MMR). We find intriguing evidence of a statistically significant
correlation between the observed distribution of compact low-mass systems and
the resonance structure, indicating a possible causal relation. While resonance
chains such as Kepler-60, Kepler-80 and TRAPPIST-1 are strong contributors,
most of the correlation appears to be caused by systems not identified as
resonance chains. Finally, we discuss their possible origin through planetary
migration during the last stages of the primordial disc and/or an eccentricity
damping process.
