The CNS3 catalog contains all known stars as of 1991 that are within 25 parsecs of the Sun. It depends mainly on a preliminary version (Spring 1989) of the new General Catalogue of Trigonometric Parallaxes (YPC) prepared by Dr. William F. van Altena (Yale University). The catalog contains every then-known star with a trigonometric parallax greater than or equal to 0.0390 arcsec, even though it may be evident from photometry or for other reasons that the star has a larger distance. For red dwarf stars, new color-magnitude calibrations for broad-band colors were carried out and applied. For white dwarfs, the recipes of McCook and Sion in ApJS, 65, 603 (1987) were applied. Stroemgren photometry was used (not yet systematically) for early-type stars and for late dwarfs, the latter supplied by E. H. Olsen from Copenhagen Observatory (private communication). In contrast to the CNS2 (Gliese 1969) catalog, trigonometric parallaxes and photometric or spectroscopic parallaxes were not combined in this version. The resulting parallax in the present version is the trigonometric parallax if the relative error of the trigonometric parallax is smaller than 14 percent. The parallax may be the photometric or spectroscopic parallax only if no trigonometric parallax is available or if the standard error of the trigonometric parallax is considerably larger. The original catalog and the notes describing it were prepared by Wilhelm Gliese and Hartmut Jahreiss, Astronomisches Rechen-Institut, Moenchhofstrasse 12-14, D-6900 Heidelberg 1, Germany, in 1991. The CDS has made a number of formatting and documentation changes in the period 1994-1997, e.g., in December 1997 it added a running number to the name for 1388 stars which had all been given the same name of "NN" in the original version, starting with "NN 3001" (for the first "NN" star in order of 1950 RA), and ending with "NN 4388" (for the last "NN" star in order of 1950 RA). This revised version of the catalog (CDS/ADC Catalog V/70A) was used by the HEASARC to create this database in February 2001. This is a service provided by NASA HEASARC .