The 3rd Planetary CubeSat Science Symposium will be held at NASA Goddard Space Flight Center on August 16-17, 2018, in B34-W150, with the participation of CubeSat/SmallSat scientists and developers. Discussions will include current missions, mission concepts, and opportunities for future mission selections. The sessions will also include panel discussions about strategic and technical aspects of planetary small satellite missions, and an afternoon poster session providing mission proposers the opportunity to meet with vendors and suppliers.