  We demonstrate a method to determine the position of single atoms in a
three-dimensional optical lattice. Atoms are sparsely loaded from a
far-off-resonant optical tweezer into a few vertical planes of a cubic optical
lattice positioned near a high-resolution microscope objective. In a single
realization of the experiment, we pin the atoms in deep lattices and then
acquire multiple fluorescence images with single-site resolution. The objective
is translated between images, bringing different lattice planes of the lattice
into focus. The applicability of our method is assessed using simulated
fluorescence images, where the atomic filling fraction in the lattice is
varied. This opens up the possibility of extending the domain of quantum
simulation using quantum gas microscopes from two to three dimensions.
