Board Thread:Lore Discussion/@comment-74.196.238.74-20150817224830/@comment-4544033-20150818013208

You could say Mercer betrayed the cause, so he isn't counted as one of the three. three seems to be a decent number, Nocturnal's gift is a fantastic set of armor plus her blessing, so if her shrine was attacked three is all that is needed.

You also have to think about recruitment and keeping a secret. Up until the events on Skyrim, people didn't think they existed, so numbers would be low on not only factors of skill, but trust.