There is only one file type that uses the .NS3 extension:
Lotus Notes 3 Database
Database file created by Lotus Notes 3.x; contains contact information, a text notes, and calendar as well as other information; used to store structured information for collaboration with other users and for exchanging data back and forth; can be stored on a central server, which keeps users' Notes databases up to date.
Lotus Notes was acquired by IBM and is referred to as IBM Lotus Notes.
Updated: May 16, 2010