Medipix3 is a CMOS pixel detector readout chip designed to be connected to a segmented semiconductor sensor. Like its predecessor, Medipix2, it acts as a camera taking images based on the number of particles which hit the pixels when the electronic shutter is open. However, Medipix3 goes much further than Medipix2 permitting colour imaging and dead time free operation. A novel charge summing and allocation scheme is implemented at the pixel level permitting proper binning of the energy of incoming photons overcoming the effects of fluorescence and charge diffusion. As there are 2 counters in each 55 pixel the chip can be programmed such that one counter is being read out while the other is counting. It is also possible to connect the chip to a sensor matrix with a pitch of 110. In this way, up to 8 counters are available per pixel.