  We experimentally study three-body energy exchange during Rydberg excitation
near a two-body F\"orster resonance. By varying the excitation pulse duration
or Rabi frequency, we control the process that causes the exchange. We
demonstrate the coherence of excitation to three-atom entangled states using an
optical rotary echo technique, and compare with a model for excitation in a
three-atom basis. Our results suggest a robust way to implement a three-body
entangling operation.
