This is all they have in the documentation:
http://wiki.bigbrotherbot.net/usage:manual_configurationb3.log_level : The amount of logging you want in your [#b3.logfile]. Lower numbers log more information and contain all the messages with higher numbers after it.
* 8 - Extra verbose debugging
* 9 - Verbose debugging
* 10 - Debugging
* 20 - Extra information
* 21 - B3 log messages
* 22 - Game log messages
* 30 - Warnings
* 40 - Errors
* 50 - Critical errors
It doesn't look like you could log only the B3 messages, but you could experiment with levels 22, 30, 40 and 50 and see what is shows.