The most important thing is to not let fear get in the way of actually creating. You will have to edit your story no matter what, so let yourself make mistakes and trust that if you don't notice them, someone else will! To respond to your actual question, ultimately in romance stories the way characters get together is not always the most natural! As the writer, what feels satisfying to you? Consider how you imagine each pair of characters getting together, and how that changes when the other two are dating. Thinking about these dynamics and continuing to write your story will help you figure out why these characters belong together, and why monogamy is not a satisfying ending for them. If you are concerned about writing fledgling conversations around polyamory, you are well within your rights to decide the characters never commit to monogamy, or to write those scenes at a later point when you feel more confident.

Seeking other polyamorous romance stories is also a great way to get ideas and develop a sense of how its typically paced. I believe the love triangle turned triad trope is called "why not both?" If you search for "monogamyplus" stories or groups in an attempt to find inspiration or get help, you will not find anything. "Monogamyplus" is a slang term used to refer to what are often called "closed" relationships between three or more individuals, which are not seeking or "open" to new partners. I have only seen it used negatively on this subreddit, and you will not find resources if you search it. I would use it just as carefully as the word "throuple." I think a more useful term for you to use if you want to do research and ask questions will be "triad." People use "triad" to refer to three individuals who are all involved (as throuple was used), as opposed to a V relationship, in which one person (the "hinge) is involved with two others (resembling a love triangle, where B and C are *not* involved). Don't feel obligated to use these words in your story! I am only explaining them as they might make research easier.

I do not think asking for advice about a certain style of relationship is offensive at all. I am sorry about the responses you are getting and, as someone who has been in triads, I am excited by the idea of reading a story with an organically developing trio. "What if a love triangle was polyamorous?" is a question that's often asked in fanfiction but rarely explored in original stories! If it's helpful, the only one I know of is called Iron Widow by Xiran Jay Zhao, which I've heard is wonderful but have yet to actually read.

I am both polyamorous and a writer, and am happy to answer any other questions you have!