You are looking for AnyDice: http://anydice.com/
Also, by the way, the actual "average array" for 4d6 drop the lowest is: 16, 14, 13, 12, 10, 9
5d6 highest 3 is: 16, 15, 14, 13, 12, 10
6d6 highest 3 is: 17, 16, 15, 14, 13, 11
7d6 highest 3 is: 17, 16, 16, 15, 14, 12
8d6 highest 3 is: 17, 17...