Welcome to the forum. I don't know how to advise you as it all just sort of happened for me after years of trying to be mono.
But I will tell you from experience, a third only shakes things up for so long before the relationship becomes just as normal and every day as being in a monogamous couple. Assuming you find the right person.
I wish you luck and I'm sure you'll get good advice here.
I'd rather be hated for who I am than loved for who I am not.