Phase 3 is what we call Analytical Planning.
Phase 3 seeks to answer the question: Given what we learned in Phase 2, how will we execute the remainder of the requirements to meet the business case from Phase 1?
The output of Phase 3 is a hierarchical backlog, non-functional requirements, and budget for the rest of the project.
This should not be as granular as a traditional project plan—there’s likely still complexity in the details of the work—but it is often more detailed than an early Agile backlog. Indeed, balancing the level of detail over time is a key success factor. And it’s important to continue pulling uncertainty forward in the project plan, not saving it for the end.
Reference class estimating at the feature and user story levels and risk-adjusted forecasting techniques are used to maximize predictability.