Welcome to our forum.
Whether you'd be called a secondary partner depends on many things ... including how you define "secondary."
I'm in an MFM "V" where the other two people are married. So I would be called secondary, except that we choose to consider all three of ourselves as primary partners. We live together, and share just about everything.
Part of being a secondary may entail having a separate domicile, not being as committed to the relationship as the "primary couple," or not sharing things like child-rearing and finance. It's really up to the three of you how you want to define yourselves.
You should get some good opportunities to rant here and get advice. Sounds like your poly relationship is going pretty well so far, but that doesn't mean there aren't any bumps along the way.
Best of luck to you, and glad you could join us!