User acceptance testing is a process of verifying that the product meets the user's needs and expectations. It is an important feedback mechanism that helps the Scrum Team validate their assumptions, inspect the value and quality of the Increment, and adapt their plans accordingly.
Scrum requires that the Increment be transparent, meaning that it is observable and understandable by anyone with a stake in the outcome. Scrum also requires that the feedback loop be short, meaning that the Scrum Team can receive and respond to feedback quickly and frequently. Therefore, it is not a good idea to have user acceptance testing done every third Sprint, because:
The Increment will not be transparent (B), meaning that the Scrum Team and the stakeholders will not have a clear and shared understanding of what has been done and what remains to be done. This may lead to misalignment, confusion, or conflict over the product vision, scope, and quality. The feedback loop is too long (B), meaning that the Scrum Team will not be able to inspect and adapt based on the user's feedback in a timely manner. This may lead to wasted effort, missed opportunities, or reduced value delivery.