Assuming you take a race with a racial +2 to Str, rolling a 16 will result in a total of 18. Anydice tells me that the odds of rolling a 16, 17, or 18 from 4d6 keep 3 is 13.04%, thus the odds of rolling at least one 16+ in your six ability scores is about 56.76%. Hardly impossible, but I would...