Well, I went to Dynadot and typed in a random 3N. In this case, 629. I found that it was available in .design, .email, .media, .services, .best, .life, .fyi, .today, .digital, .chat and others. So, yes there are 3N's available. Whether you should register any of these names is a different question. I would suggest not.
I know you can buy them in other extensions other than net org like Shackleton I suggest you refrain from ''investing"" also in these but hey I have been wrong. I suggest you look for something of type in value if looking at gTLD.