> x = [[1,2,3],[4,5,6],[7,8,9]]
=> [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
> x.inject(&:product).map(&:flatten)
=> [[1, 4, 7], [1, 4, 8], [1, 4, 9], [1, 5, 7], [1, 5, 8], [1, 5, 9],
[1, 6, 7], [1, 6, 8], [1, 6, 9], [2, 4, 7], [2, 4, 8], [2, 4, 9],
[2, 5, 7], [2, 5, 8], [2, 5, 9], [2, 6, 7], [2, 6, 8], [2, 6, 9],
[3, 4, 7], [3, 4, 8], [3, 4, 9], [3, 5, 7], [3, 5, 8], [3, 5, 9],
[3, 6, 7], [3, 6, 8], [3, 6, 9]]

あってる?