Welcome to our forum.
It always amazes me how society overall seems to accept cheating (which isn't honest) more easily than polyamory (which is honest). It's just conditioning, I suppose, and most people have that conditioning to overcome.
Perhaps you and N could have a talk sometime and see if you can come to a mutual agreement about where you'd like your relationship to go. Set some goals even, perhaps. I assume S knows about N and is okay with it? Always double-check any assumptions, no matter how obvious something seems.
There is a lot of potential for you (and S, maybe N as well) to find a lot of happiness through polyamory. Just take it slow, and learn as much as you can along the way.
Glad you could be with us.
Love means never having to say, "Put down that meat cleaver!"