A043515 Numbers n such that number of 6's in base 10 is 3.

666, 1666, 2666, 3666, 4666, 5666, 6066, 6166, 6266, 6366, 6466, 6566, 6606, 6616, 6626, 6636, 6646, 6656, 6660, 6661, 6662, 6663, 6664, 6665, 6667, 6668, 6669, 6676, 6686, 6696, 6766, 6866, 6966, 7666, 8666, 9666, 10666