input: Write a function to connect to three different databases: MySQL, MongoDB, and PostgreSQL, query data from each, and render the results on a web page. The function should establish connections to the databases using provided credentials, define specific queries for each database, execute the queries, close the connections, and return the results to be rendered on the web page.
  
  def display_data_from_databases():
      import pymysql
      from pymongo import MongoClient
      import psycopg2
  
      # connect to MySQL
      connection_mysql = pymysql.connect(host='localhost', user='root', password='password', database='db_name')
  
      # connect to MongoDB
      client = MongoClient('mongodb://localhost:27017')
  
      # connect to PostgreSQL
      connection_postgresql = psycopg2.connect(host='localhost', user='postgres', password='password', dbname='db_name')
  
      # define a query for each database
      query_mysql = 'SELECT * FROM table_name'
      query_mongodb = {'name': 'John Doe'}
      query_postgresql =  'SELECT * FROM table_name WHERE age > 18'
  
      # execute the query and fetch the results
      result_mysql = connection_mysql.execute(query_mysql)
      result_mongodb = client.db_name.collection_name.find(query_mongodb)
      result_postgresql = connection_postgresql.execute(query_postgresql)
  
      # close the connections
      connection_mysql.close()
      client.close() 
      connection_postgresql.close()
  
      # return the results
      return {
          'data_mysql': result_mysql, 
          'data_mongodb': result_mongodb,
          'data_postgresql': result_postgresql
      }
  
  # assert test cases
  assert display_data_from_databases()['data_mysql'] != None
  assert display_data_from_databases()['data_mongodb'] != None
  assert display_data_from_databases()['data_postgresql'] != None
output: 
  def display_data_from_databases():
      import pymysql
      from pymongo import MongoClient
      import psycopg2
  
      # connect to MySQL
      connection_mysql = pymysql.connect(host='localhost', user='root', password='password', database='db_name')
      cursor_mysql = connection_mysql.cursor()
  
      # connect to MongoDB
      client = MongoClient('mongodb://localhost:27017')
      db_mongo = client.db_name
      collection_mongo = db_mongo.collection_name
  
      # connect to PostgreSQL
      connection_postgresql = psycopg2.connect(host='localhost', user='postgres', password='password', dbname='db_name')
      cursor_postgresql = connection_postgresql.cursor()
  
      # define a query for each database
      query_mysql = 'SELECT * FROM table_name'
      query_mongodb = {'name': 'John Doe'}
      query_postgresql =  'SELECT * FROM table_name WHERE age > 18'
  
      # execute the query and fetch the results
      cursor_mysql.execute(query_mysql)
      result_mysql = cursor_mysql.fetchall()
  
      result_mongodb = list(collection_mongo.find(query_mongodb))
  
      cursor_postgresql.execute(query_postgresql)
      result_postgresql = cursor_postgresql.fetchall()
  
      # close the connections
      cursor_mysql.close()
      connection_mysql.close()
      client.close() 
      cursor_postgresql.close()
      connection_postgresql.close()
  
      # return the results
      return {
          'data_mysql': result_mysql, 
          'data_mongodb': result_mongodb,
          'data_postgresql': result_postgresql
      }
  