Should I np 3 Nero?

Yes! there is no point in having 3 of the same servant(usually) plus the jump between NP1 and NP2 is huge, combine that with the already strong NP that Nero has will make her one of your best servants.
UMU>MIKON

Yes. If you intend on using a servant, it is always worth it to up their NP.

What would the alternative even be? You can't add multiple copies of the same servant to your party, AFAIK, so there's no point in keeping multiple copies of her. I guess you could burn them if you really want some rare prisms, but she's probably too good to burn, plus she's a limited servant.

Rule of thumb is usually yes it's always worth NP upgrading any servant.

The NP boost is always appreciated even with the most niche servant. I got spooked by Stheno twice and while I haven't really invested in her, I know her NP2 self will be useful in certain content especially if I were to do a charm-lock stall team.

Soon you will want duplicate servants more than some new servants