  The quantum-mechanical three-body problem is one of the fundamental
challenges of few-body physics. When the two-body interactions become resonant,
an infinite series of universal three-body bound states is predicted to occur,
whose properties are determined by the strength of the two-body interactions.
We report on the association and direct observation of a trimer state
consisting of three distinguishable fermions using radio-frequency (RF)
spectroscopy. The measurements of its binding energy are consistent with
theoretical predictions which include non-universal corrections.
