How to insert number every nth letter in a string in Python?

I'm writing a program to encrypt a string input. I have a random number generator, and some code that converts the random number into a letter. How would I go about inserting this letter after every say, 3rd letter? I.e. String before: abcdef , String after: abcldefk.

Code for the random number generator if it helps:

Letter = random.randrange(1,26) 

print chr(Letter + ord('A'))