  We investigate the interaction between three rubidium atoms in highly excited
($58p$) Rydberg states lying along a common axis and calculate the potential
energy surfaces (PES) between the three atoms. We find that three-body
long-range potential wells exist in some of these surfaces, indicating the
existence of very extended bound states that we label \textit{macrotrimers}. We
calculate the lowest vibrational eigenmodes and the resulting energy levels and
show that the corresponding vibrational periods are rapid enough to be detected
spectroscopically.
