Pen name or pseudonym (3,2,5)
I believe the answer is:
nom de plume
I believe this is a double definition.
'pen name' is the first definition.
(synonyms)
'pseudonym' is the second definition.
(nom de plume is a kind of pseudonym)
(Other definitions for nom de plume that I've seen before include "Assumed name" , "Pen-name" , "False name" , "Author's pseudonym" , "Pen name" .)