Yay for ugly, inefficient one-liners!

ruby_array_insert_every.rb

# Using insertion
class Array
  # Insert into the array.
  def insert_every!(skip, str)
    (skip...size).step(skip).with_index {|n, i| insert(n + i, str) } 
    self
  end

  # Create a new array and insert into it.
  def insert_every(skip, str)
    dup.insert_every! skip, str
  end
end

p %w[ 1 2 3 4 5 6 7 8 9 10 11 12 ].insert_every!(3, ',')
#=> ["1", "2", "3", ",", "4", "5", "6", ",", "7", "8", "9", ",", "10", "11", "12"]

p %w[ 1 2 3 4 5 6 7 8 9 10 11 12 ].insert_every(3, ',')
#=> ["1", "2", "3", ",", "4", "5", "6", ",", "7", "8", "9", ",", "10", "11", "12"]

# Using a crazy chain of fun!
class Array
  # Create a new array and insert into it.
  def insert_every(skip, str)
    each_slice(skip).map {|a| a << str }.to_a.tap {|a| a.last.pop }.flatten(1)
  end
end

p %w[ 1 2 3 4 5 6 7 8 9 10 11 12 ].insert_every(3, ',')
#=> ["1", "2", "3", ",", "4", "5", "6", ",", "7", "8", "9", ",", "10", "11", "12"]