“Jim Womack” refers to “Purpose, Process & People” time & again when expounding LEAN thinking.  ( One such note is  here).  To explain briefly …

Purpose is about, what is the Value / outcome expected / Why  / Need ?

Process – Do our processes serve the Purpose, are they …
               Valuable — Is the Output useful
               Capable – Get the required result consistently
                Available – Can be executed as and when needed
                Adequate – Does it meet the required Quantitative needs
                Flexible – Can it accommodate variations, can be quickly reconfigured. to changing needs.

People – Do we have engaged & Motivated people Skilled & Competent in executing the process

Jim explains that by asking these questions during Gemba walks, aligning the teams thought process to LEAN thinking, is triggered.

(This was an ultra brief summary … let’s get to the main part)

Now, If we have just conceptualized a project and need to plan the execution.
Purpose is about understanding the Needs and Identifying the Scope & its boundaries.
By explicitly questioning if the Selected Scope serves the true Purpose, we are bringing in alignment with the Customer Needs.

Process is about the Execution plan the WBS / Development Life-cycle / Intermediate Milestones… etc. The good part is we get to think through the abilities (Valuable / Capable / Available / Adequate / Flexible) of each of the Sub processes / Activities. this would help in strengthening the espoused Abilities

Focus on People is about having a team with the required Competencies / Skills, and developing plans to acquire them. Providing for an environment to maximize team engagement towards attaining the purpose.

When we have Uncertainties associated with the 3P’s those are identified as Program Risks. Then we develop plan to mitigate them to reduce the uncertainties.

When we have gaps between the expected outcome & purpose, or the planned processes are not adequately executed, those result in product or process Quality issues .

Thus approaching Project Planning with 3P’s thinking helps improve the Planning effectiveness.