The 3rd Edition of the Multifrequency Catalogue of Blazars is one of the most complete lists of Active Galactic Nuclei whose emission properties are recognised as typical of blazars. It includes the list of sources and an essential compilation of multifrequency data from radio to gamma rays. This catalogue of blazars was initially assembled with the aim of providing a useful tool for the identification of the counterparts of the many gamma-ray sources detected by Fermi-GST, and it was actually used by the Fermi-LAT team for the preparation of the 1 LAC and 2 LAC catalogues. However, blazars are important observational targets for a number of present and future space missions and ground-based observatories. The source list for the entire sky is also available online at the ASDC web site where it is frequently updated to add new blazars and to improve the database.