test.rb

    (0..3).each do |x|
      diag_array = []
      3.times { |num| diag_array  << flat_grid[x + (num)*7] }
      arrays << diag_array
    end
    (7..10).each do |x|
      diag_array = []
      3.times { |num| diag_array  << flat_grid[x + (num)*7] }
      arrays << diag_array
    end
    (14..17).each do |x|
      diag_array = []
      3.times { |num| diag_array  << flat_grid[x + (num)*7] }
      arrays << diag_array
    end

arrays << [*0..3, *7..10, *14..17].map { |n| flat_grid[x + n * 7] }