postgis - How to calculate the average distance among set of points as measure of closeness

I have three series (or tables) of points in PostGIS which I want to compare. I would like to know how "close" each series is when compared to the other two. In other words I would need some measure such as the average of the average distance of each point with all the others (that is, the average of the distance matrix)... Sounds complicated, but I am sure some common statistic exist to compare series of points.

How my query would look like?