To build web applications using Python 3, you can use the Flask framework. Flask is lightweight, flexible, and great for creating web apps quickly. Here are the steps to get started:

Install Flask:
First, activate your Python environment and install Flask using pip:

   source env/bin/activate  # Activate your environment
   pip install flask

Create a Simple Flask Application:
Create a Python file (e.g., app.py) and write a basic Flask app:

   from flask import Flask

   app = Flask(__name__)

   @app.route('/')
   def hello():
       return 'Hello, World!'

   if __name__ == '__main__':
       app.run(debug=True)

Run the Application:
Run your Flask app:

   python app.py

Routes and View Functions:
Define routes (URL paths) and view functions (what to display for each route). For example:

   @app.route('/about')
   def about():
       return 'About page'

   @app.route('/contact')
   def contact():
       return 'Contact us'

Dynamic Routes:
Create dynamic routes that accept parameters (e.g., user IDs):

   @app.route('/user/')
   def user_profile(user_id):
       return f'User {user_id} profile'

Debugging:
Enable debug mode during development for helpful error messages:

   app.run(debug=True)

Remember to adapt this example to your specific project.