  Stable bound quantum states are ubiquitous in nature. Mostly, they result
from the interaction of only pairs of particles, so called two-body
interactions, even when large complex many-particle structures are formed. We
show that three-particle bound states occur in a generic, experimentally
accessible solid state system: antiferromagnetic spin ladders, related to
high-temperature superconductors. This binding is induced by genuine
three-particle interactions; without them there is no bound state. We compute
the dynamic exchange structure factor required for the experimental detection
of the predicted state by resonant inelastic X-ray scattering for realistic
material parameters. Our work enables us to quantify these elusive interactions
and unambiguously establishes their effect on the dynamics of the quantum
many-particle state.
