Hi asker302,
I would not recommend you to leave it blank. strictly follow the guidelines provided by Citizenship form guide and you will have peace of mind.
If you do bit of research on the forums, i found that moment you enter anything in 3B and 3C they have got letter with enquiry for proof of documentation regarding the name written there as 3B and 3C is for Name Change cases generally applicable to women changing their last name after marriage.
The cases were the application got enquiry letter later submitted to withdrawn change of name and then there application was processed just causing delay in application.
If you still have doubt, best to Call CIC call centre and follow their directions. The agents are more than happy to assist.