Rashi on Deuteronomy 27:2/רש"י על דברים כ״ז:ב׳

1והקמת לך THEN THOU SHALT RAISE THEE UP [GREAT STONES] — in the Jordan, and afterwards you shall take out from there others and build an altar of them on Mount Eval. Consequently you must say that there were three sets of stones: twelve in the Jordan, an equal number in Gilgal, and another twelve on Mount Ebal, as is stated in Treatise Sotah 35b.