Produces the terminal flavan-3-ol monomers required for the formation of proanthocyanidins or condensed tannins in leaves and flowers, as well as in the skin and seeds of developing berries. Behaves as a reductase and as a C-3 epimerase. Catalyzes the double reduction of anthocyanidins, producing a mixture of (2S,3S)- and (2S,3R)-flavan-3-ols. The enzyme catalyzes sequential hydride transfers to C-2 and C-4, respectively and epimerization at C-3 is achieved by tautomerization that occurs between the two hydride transfers. Converts cyanidin, pelargonidin and delphinidin into catechin and epicatechin, afzelechin and epiafzelechin, and gallocatechin and epigallocatechin respectively