require 'csv'

letters = ['a', 'b', 'c']
numbers = [1,2,3]
numbers_2 = [4,5,6]

csv_string = CSV.generate do |csv|
  letters.each_with_index do |letter, index|
    csv << [
      letter,
      numbers[index],
      numbers_2[index]
    ]
  end
end

puts csv_string