The first one is usually less crowded, because people typically like to wait until after dark to go. (But if you go to the first one, the sun slowly sets as the processional goes on. Kinda nice!)
This is sort of an impossible question to answer. We typically go to three or four CPs each year and have never been to one that wasn't completely full.
As for queue length though - the third show will typically be the shortest, though not by much.