input: from typing import List
  
  def visualize_twitter_data(twitter_data: List[dict]):
      """ Given a list of dictionaries representing Twitter data, each containing fields like 'author', 'date', 'text', 'likes', 'retweets',
      this function visualizes the data in three bar charts: one showing the number of tweets per author, one showing the number of likes per tweet,
      and one showing the number of retweets per tweet.
      
      >>> visualize_twitter_data([{'author': 'user1', 'date': '2023-01-01', 'text': 'Hello', 'likes': 10, 'retweets': 5},
      ...                        {'author': 'user2', 'date': '2023-01-02', 'text': 'Hi', 'likes': 15, 'retweets': 3},
      ...                        {'author': 'user1', 'date': '2023-01-03', 'text': 'Good morning', 'likes': 20, 'retweets': 7}])
      # The function will generate three bar charts as described in the docstring.
      """
output:     import matplotlib.pyplot as plt
  
      authors = [tweet['author'] for tweet in twitter_data]
      likes = [tweet['likes'] for tweet in twitter_data]
      retweets = [tweet['retweets'] for tweet in twitter_data]
  
      # Bar chart for number of tweets per author
      author_counts = {author: authors.count(author) for author in set(authors)}
      plt.figure(figsize=(10, 5))
      plt.bar(author_counts.keys(), author_counts.values(), color='blue')
      plt.xlabel('Author')
      plt.ylabel('Number of Tweets')
      plt.title('Number of Tweets per Author')
      plt.show()
  
      # Bar chart for number of likes per tweet
      plt.figure(figsize=(10, 5))
      plt.bar(range(len(likes)), likes, color='green')
      plt.xlabel('Tweet Index')
      plt.ylabel('Number of Likes')
      plt.title('Number of Likes per Tweet')
      plt.show()
  
      # Bar chart for number of retweets per tweet
      plt.figure(figsize=(10, 5))
      plt.bar(range(len(retweets)), retweets, color='red')
      plt.xlabel('Tweet Index')
      plt.ylabel('Number of Retweets')
      plt.title('Number of Retweets per Tweet')
      plt.show()