I am no “Know-it-all” especially when it comes to wormholes which I have never ratted in yet. But I do alot of “normal” ratting.
I read somewhere that a Nightmare might be a bit overkill for a C3 since the Sleeper-Rats there are mostly cruiser and frig size. Don’t quote me on that one though because as written above I never been there personally.
A T3 cruiser seems to be a solid choice as far as I heard from other players. Linking a good fit though is a bit complicated since most fits that you could link here are still based on the old subsystem set. And I don’t have a Legion fit ready for you at the moment.
You could use O.smium.org to search for a “Legion C3” or something alike and work with the ingame fitting simulator or one of the offline tools to adapt that fitting to your needs and skills. I think that would be a good idea. I use Pyfa to work on my fittings. It is a very useful tool once you get used to it.
One point I’d like to advice you on besides ship and fitting is this:
No matter your ship and fitting though you always have to be aware of other players in wormholes. Rely on D-Scan and or combat probes to check if a system is already beeing “taken” by a corp. Scout a WH-system out before you take your more or less expensive ratting ship(s) in there. Since you have no local channel you never really know if you are “alone” or not. Always keep this in mind when you rat in a WH-system otherwise you will become the victim of other players who will most likely kill you with overwhelming force since “fair play” is something you rarely see in PVP in EVE.
End of Wall-o-text incoming. I think you will find a decent T3 Cruiser fitting on O.Smium or maybe even here that you can build on. Have fun and fly safe.