Using Google I found, as I expected, that it is available from le Centre des Donnees at Strasbourg. See http://cdsarc.u-strasbg.fr/viz-bin/Cat?cat=I%2F315& The huge number of files (over 2Gbytes total) can be downloaded via the FTP tab on that page. However Googling also led to something which suggests that Astrometrica simply gets small amounts of data as required by HTTP requests to Vizier (also part of the Strasbourg site) so it should not be necessary to download the full data set.
Peter,Is there any reason why you want to download the complete UCAC-3 catalogue? As Graham says Astrometrica will download the stars that it needs from VizieR when you reduce an image. This is a lot more efficient that downloading the whole catalogue although it does mean that you need to be online when you are reducing images. Just select VizieR on the settings->Catalogs tab and select an appropriate VizieR server on the Internet tab (I use CDS, Strasbourg).Nick.