..., two special tags are always available: `HEAD' refers to the most recent version available in the repository, and `BASE' refers to the revision you last checked out into the current working direc- tory.
と、manにはあるのですが…
#cvs rdiff -r eda -r HEAD modulename、または #cvs diff -Dtodayとやるとb3とa3の差分が出るようです。