input: Write a python function to connect to MySQL, MongoDB, and PostgreSQL databases, execute specific queries, and render the results on a webpage.
  assert connect_and_query_databases() == {'data_mysql': result_mysql, 'data_mongodb': result_mongodb, 'data_postgresql': result_postgresql}
output: def connect_and_query_databases():
      import mysql.connector
      import pymongo
      import psycopg2
      from flask import Flask, render_template_string
  
      app = Flask(__name__)
  
      # MySQL connection and query
      def query_mysql():
          conn = mysql.connector.connect(
              host="your_mysql_host",
              user="your_mysql_user",
              password="your_mysql_password",
              database="your_mysql_database"
          )
          cursor = conn.cursor()
          cursor.execute("your_mysql_query")
          result = cursor.fetchall()
          conn.close()
          return result
  
      # MongoDB connection and query
      def query_mongodb():
          client = pymongo.MongoClient("your_mongodb_uri")
          db = client.your_mongodb_database
          result = list(db.your_mongodb_collection.find("your_mongodb_query"))
          client.close()
          return result
  
      # PostgreSQL connection and query
      def query_postgresql():
          conn = psycopg2.connect(
              host="your_postgresql_host",
              user="your_postgresql_user",
              password="your_postgresql_password",
              database="your_postgresql_database"
          )
          cursor = conn.cursor()
          cursor.execute("your_postgresql_query")
          result = cursor.fetchall()
          conn.close()
          return result
  
      # Execute queries
      result_mysql = query_mysql()
      result_mongodb = query_mongodb()
      result_postgresql = query_postgresql()
  
      # Rendering results on a webpage
      @app.route('/')
      def index():
          return render_template_string('''
          <h1>Database Query Results</h1>
          <h2>MySQL Results</h2>
          <pre>{{ data_mysql }}</pre>
          <h2>MongoDB Results</h2>
          <pre>{{ data_mongodb }}</pre>
          <h2>PostgreSQL Results</h2>
          <pre>{{ data_postgresql }}</pre>
          ''', data_mysql=result_mysql, data_mongodb=result_mongodb, data_postgresql=result_postgresql)
  
      app.run()
  
      return {'data_mysql': result_mysql, 'data_mongodb': result_mongodb, 'data_postgresql': result_postgresql}