What are the 3 requirements for fusion?

Can you please elaborate on the three crucial requirements for achieving fusion? I'm particularly interested in understanding how these requirements work together to facilitate the process. Specifically, I'd like to know what the first requirement entails, how it sets the foundation for the subsequent steps, and what specific challenges or considerations need to be addressed in order to satisfy this criterion. Furthermore, I'm curious about the second and third requirements and how they build upon the first, creating a comprehensive framework for fusion to occur. Thank you for your insights.