input: Write a function to connect to three different databases (MySQL, MongoDB, and PostgreSQL), execute a query on each, and display the results on a web page. If the connections or queries fail, return an error message.
  
  import pymysql
  from pymongo import MongoClient
  import psycopg2
  from django.shortcuts import render
  from django.http import HttpResponse
  
  def query_databases():
      try:
          # 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
          cursor_mysql = connection_mysql.cursor(pymysql.cursors.DictCursor)
          cursor_mysql.execute(query_mysql)
          result_mysql = cursor_mysql.fetchall()
  
          result_mongodb = client.db_name.collection_name.find(query_mongodb)
          result_mongodb = list(result_mongodb)
  
          cursor_postgresql = connection_postgresql.cursor(cursor_factory=psycopg2.extras.RealDictCursor)
          cursor_postgresql.execute(query_postgresql)
          result_postgresql = cursor_postgresql.fetchall()
  
          # close the connections
          connection_mysql.close()
          client.close() 
          connection_postgresql.close()
  
          return result_mysql, result_mongodb, result_postgresql
  
      except Exception as e:
          return str(e)
  
  def render_page(request):
      results = query_databases()
      if isinstance(results, str):
          return HttpResponse(results)
      else:
          result_mysql, result_mongodb, result_postgresql = results
          return render(request, 'template.html', {
              'data_mysql': result_mysql, 
              'data_mongodb': result_mongodb,
              'data_postgresql': result_postgresql
          })
  
  assert query_databases()[0] is not None
  assert len(query_databases()[1]) > 0
  assert len(query_databases()[2]) > 0
output: 
  def render_page(request):
      results = query_databases()
      if isinstance(results, str):
          return HttpResponse(results)
      else:
          result_mysql, result_mongodb, result_postgresql = results
          return render(request, 'template.html', {
              'data_mysql': result_mysql, 
              'data_mongodb': result_mongodb,
              'data_postgresql': result_postgresql
          })
  