M. L.  Ekiel-Jezewska, T. Gubiec, and P. Szymczak, Phys. Fluids 20, 063102 (2008)                   

Periodic motion of three identical spheres settling under gravity in Stokes flow. Left: in the center-of-mass frame. Right: in the frame of reference moving with the red sphere.  The gravity points vertically down.