Why the process validation batch must be 3?
Neither FDA nor any other regulation specifies the maximum number of batches to be considered as validation. The manufacturers have to choose the number of batches to be validate in this regard. The number of batches to be taken under validation depends upon the risk involved in the process of manufacturing. The less knowledge about the process requires the more statistical data to confirm the consistent
performance. Consideration of validation batches fewer than three will require more statistical and scientific data to prove the consistency of process to meet quality standards.
FDA’s “Guidance for Industry on Process Validation: General Principles and Practices” provides the guideline for process validation, no longer consider the traditional three batch validation appropriate but also does not prescribe the number of batches to validate or suggests any other method to determine it. Ref;https://www.pharmaguideline.com/2013/08/three-consecutive-batches-for-validation.html