Projects
Projects are temporary. They have a start and an end date. Projects deliver a new product, a service, a result or business value.
Example – Developing a new website is a project
Programs
Programs are commonly defined as a group of related projects that provide certain benefits when managed together.
Example – Managing the development of all ecommerce projects together can be a program

Portfolios
Portfolios are not just a group of programs but can also include projects that do not align with any program.
Example – Managing all types of website development related projects and programs can be a portfolio while managing all mobile apps related projects and programs can be another portfolio
Check more articles on Project Management Basics