  Three-body F\"orster resonances at long-range interactions of Rydberg atoms
were first predicted and observed in Cs Rydberg atoms by R.Faoro et al., Nature
Comm. 6, 8173 (2015). In these resonances, one of the atoms carries away an
energy excess preventing the two-body resonance, leading thus to a Borromean
type of F\"orster energy transfer. But they were in fact observed as the
average signal for the large number of atoms $N\gg1$. In this Letter we report
on the first experimental observation of the three-body F\"orster resonances
${\rm 3}\times nP_{3/2} (|{\rm M}|)\to nS_{1/2} +(n+1)S_{1/2} +nP_{3/2} (|{\rm
M}^{*} |)$ in a few Rb Rydberg atoms with $n=36, 37$. We have found here clear
evidence that there is no signature of the three-body F\"orster resonance for
exactly two interacting Rydberg atoms, while it is present for $N=3-5$ atoms.
This demonstrates the assumption that three-body resonances can generalize to
any Rydberg atom. As such resonance represents an effective three-body
operator, it can be used to directly control the three-body interactions in
quantum simulations and quantum information processing with Rydberg atoms.
