from random import choice


string = "Hello bitches, it's let"

def get_alphas(string):
  return ''.join(filter(str.isalpha, string))

def star_replace(string: str, count: int) -> str:
  if count > len(get_alphas(string)):
    raise ValueError("count can't be bigger then count of alphabeth symbols in string")

  for _ in range(count):
    string = string.replace(choice(get_alphas(string)), '*', 1)
  return string

print(star_replace(string, 3))