An entity is said to be in the third normal form if it is already in 2NF and no non-identifying attributes are dependent on any other non-identifying attributes.
Attributes that are dependent on other non-identifying attributes are normalized by moving both the dependent attribute and the attribute on which it is dependent into a new entity.
The data model in 3NF
When a data model is in 3NF it has been normalized.
There are other normal forms that have some value. For most design purposes, however, the third normal form is sufficient to guarantee a proper design.