  We show that Resonant Inelastic X-ray scattering (RIXS) is sensitive to
three-magnon excitations in cuprates. Even if it requires three electrons to
simultaneously flip their spin, the RIXS tri-magnon scattering amplitude is not
small. At the Cu $L$-edge its intensity is generally larger than the bi-magnon
one and at low transferred momentum even larger than the single-magnon
intensity. At the copper $M$-edge the situation is yet more extreme: in this
case three-magnon scattering is dominating over all other magnetic channels.
