So, since you have 3 different possible character sets to choose from, I made a simple r variable that makes a random choice between 1 and 3, and picks a character from a character set based on that choice.

I made a fork of your repl, with the modifications I listed above, you can look at: https://repl.it/@heyitsmarcus/Password-Generator

import random
print ("""This is my simple password generator.""")
letters = "abcdefghijklmnopqrstuvwxyz"
symbols = "[email protected]#[email protected]*"
numbers = "123456789"
length = int(input("How long do you want your password? "))
password = ""
for c in range(length):
  r = random.randint(1,3)
  if (r == 1):
  	password += random.choice(letters)
  elif (r == 2):
	  password += random.choice(symbols)
  elif (r == 3):
	  password += random.choice(numbers)
print (password)

Um it works for me... Just for c in length(length//3): so the password is the right length.

By the way, I recommend DiceWare as a password generator. It makes a memorable password.