Interaction Framework can work with or without creating a project-to-project reference; however, typically a multi-project solution is deployed to the user as one package containing all projects. Therefore, using the project reference with Interaction Framework ensures that the framework works as one unit for the solution and not by project.
As per the above statement it is not require to create project to project reference.
If there are 3 projects how these 3 projects can connects and communicate with each other.
As per my knowledge , for projects to communicate and pass data among project we need Interaction framework and project-to-project reference is required.