Multistage deterministic lateral displacement devices, methods of forming the devices, and methods of separating a fluid mixture including particles having three or more particle sizes generally include a first module and at least one additional module. Each module includes a condenser portion and a separate portion. The condenser portion is generally configured to focus a streamline of all particles to a center of a channel whereas the separator separates the streamline of all particles into two different streamlines. One of the streamlines focuses the largest particles in the fluid mixture along a sidewall of the channel and the other streamline of smaller particles is between opposing sidewalls that define the channel. Each additional module can be used to further separate the largest particles remaining in the fluid mixture from the smaller particles.