  We report on three-dimensional (3D) electron momentum distributions from
single ionization of helium by a laser pulse consisting of two counterrotating
circularly polarized fields (390 nm and 780 nm). A pronounced 3D low energy
structure and sub-cycle interferences are observed experimentally and
reproduced numerically using a trajectory based semi-classical simulation. The
orientation of the low energy structure in the polarization plane is verified
by numerical simulations solving the time dependent Schr\"odinger equation.
