Hi Diana! Thanks for reaching out. These are practically three different chronologically-related steps in the entire analytic process, so to speak.  The first step is to do a descriptive analysis. It provides basic information such as the distribution of your data, a first check on the quality of your data (e.g. see if there's completely erroneous information, missing data etc.), and helps you see if there are certain patterns deriving from your data that you could explore in the next steps of the analysis. Diagnostic analysis is namely the next step. It involves the use of statistical techniques to explain the behaviour and patterns obtained from the provided data. I.e. this step of the analysis regards "looking to the past" to see what has already happened and study it with greater precision. Then comes predictive analytics. It is about using the same or different techniques compared to the ones used during the diagnostic analysis. However, it regards predicting the future behaviour of the business. Hope this helps.
Best,
Martin