The 6s could be in positions 2 and 3: 066, 166, 266, 366, 466, 566, 666, 766
The 6s could be in positions 1 and 3: 606, 616, 626, 636, 646, 666, 676, 686, 696
The 6s could be in positions 1 and 2: 660, 661, 662, 663, 664, 665, 666, 667, 668, 669
Count them (just don't count repeats) and make certain that I didn't forget any ...