What are the exact differences between EC2, Beanstalk and LightSail in AWS?
What are good real time scenarios in which I should use these services?
They are all based on EC2, the compute service from AWS allowing you to create EC2 instances (virtual machines in the cloud).
Lightsail is packaged in a similar way than Virtual Private Server, making it easy for anyone to start with their own server. It has a simplified management console and many options are tuned with default values that maximize availability and security.
Elastic Beanstalk is a service for application developers that provisions an EC2 instance and a load balancer automatically. It creates the EC2 instance, it installs an execution environment on these machines and will deploy your application for you (Elastic Beanstalk support Java, Node, Python, Docker and many others)
Behind the scenes, Elastic Beanstalk creates regular EC2 instances that you will see in your AWS Console.
And EC2 is the bare service that allows the other to be possible. If you choose to create an EC2 instance, you will have to choose your operating system, manage your ssh key, install your application runtime and configure security settings by yourself. You have full control of that virtual machine.