The Three Amigos refers to the lead developer, qa analyst, and business representative reviewing and elaborating the requirements. They should be taking the business requirements and turning them into real acceptance criteria. That acceptance criteria should then be used to drive testing at all levels.
Hopefully, the acceptance criteria is written in a way that can be directly translated into tests. One technique that can be used is to write the acceptance criteria using a Given-When-Then syntax. Given is the precondition, When is the action, Then is the result after the action is complete.
Mockups and screenshots are a great way to provide some high level detail about the change. Where it falls short is in really shaking out the details, providing specifics, and defining behaviors for edge cases.