NT-3 is a good vocal mic on the right voice... The general rule is that you use a soft mic for a hard voice and vice versa. As the NT-3 is a bright,edgey mic, a soft voice is it's best use.
I own several NT-3's as well as a K2 and NT-2. They all get used on vocals at various times depending on, as I said, the voice. Dynamic mics are good too for strong vocal styles as stated earlier... but good preamp is a must.
At the end of the day every voice has it's own best mic and you need to try lots to find the best.