I wouldn't know any AF Analysis function or combination of functions which would return the average of the 3 highest values for a certain period. The next version of Asset Analytics introduces support for arrays, please see the PI System Roadmap for PI-AF, and this may change the picture.
Today I would use a programmatic approach by retrieving values through AF SDK and doing the analysis client side.
The array functions Gregor Beck mentioned can be previewed here: Sneak Peek at AF 2018 with Asset Analytics: Arrays
This is somewhat of a common use case and for those who required the result stored in PI, people either write custom code that is scheduled or determine the top N values using PI OLEDB Enterprise along with a linked table. I've done this using our product EdictVB.Net, or just on demand client side via a custom reporting solution.