NV3 (Novice/Recruit/Cadet) is the third rank in the Dark Brotherhood, which is awarded after completing the requirements of a NV2 (Apprentice).
It is part of the Novitiate class, the third step down the path to NV4 (Proselyte/Trainee/Trooper).
The requirements for the promotion from NV3 are found at the above link, the completion of the above requirements will earn a promotion to NV4 (Proselyte/Trainee/Trooper).