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.

A new Sprint starts immediately after the conclusion of the previous Sprint. Delaying the start of the next Sprint interrupts progress and can reduce the Time-to-Market. Shortening the Sprint length allows the team to respond quicker to changes outside of the Sprint plan and while still allowing delivery of business value. Adding user feedback to the Product Backlog helps the Product Owner with ordering the backlog to maximize the flow of value.
During the Sprint no changes are made that would endanger the Sprint Goal.

This is the best answer because it is consistent with the Scrum Master's accountability to educate and coach the Scrum Team and the organization on the Scrum framework and its principles12. By coaching the Product Owner and the Developers on the purpose of Scrum events, the Scrum Master can help them understand that the Daily Scrum is not a status report or a performance review, but a time to inspect and adapt their progress toward the Sprint Goal34. By letting them figure out how to deal with this situation, the Scrum Master can also respect and enable their self-organization and collaboration12.

The ones that will be doing the work are the best ones to decide on what Development Team structures work best.

The Scrum Master is responsible not only for coaching the Scrum Team but also the organization.