h h This third volume of RC3 indudes data for galaxies between 12 and 24 . For convenience, we repeat the explanation of the catalogue entries below. Details of the reduction procedures, and Notes, References, and Appendices are in Volume 1. 2. The Catalogue The data for each galaxy are found on four successive lines on a single page. The entries are as follows: Column 1: Positions Line 1: RA and DEC = right ascension and dedination for the equinox 2000.0, precessed from the 1950.0 position in Column 1, Line 4, given to 0.1 second of time and 1 arcsec when available, and to 0.1 minute of time...