A three million pixel photon counting large area detector has been commissioned for the coherent diffraction imaging beamline (I13) of Diamond Light Source. Challenging requirements of small pixel size, large area and fast frame rates required the construction of a detector based upon a tiling of a large number of the latest generation of the Medipix family of photon counting ASICs, Medipix3RX and a highly parallel readout system based upon the Front End Module cards developed by STFC. Such a detector provides a significant improvement in field of view whilst maintaining the resolution afforded by the small pixel size, when compared to previously available Medipix-based detectors such as the quad-chip Merlin. A number of commissioning experiments have been performed on the beamline, and this Conference Record provides intial results from these measurements and determines the capabilities of the detector and beamline system.