Partner wants to be polyamorous, but I'm not sure

[An-Insect]

So, starting with context, call me Bugs. I'm a 17 y/o trans man who, a couple of weeks ago, entered an online relationship with another 17 y/o trans man, who I'll refer to as M. While I say "online" the situation is a bit more complicated than that and he'll actually be moving in with me in January. This is my second ever romantic relationship, the previous one lasted for 3 years and ended last November.
Earlier today, M told me he just found out a friend of his (17, it/he/they) has a crush on him, and that M is also interested in that friend. I also know this friend, but not very much. M asked for my thoughts on polyamory and if it would work for our relationship, to which I said I needed some time to consider it.

On one hand; the idea of having multiple romantic partners sounds nice, the friend seems like a cool person, M has known them for much longer than he's known me, and I do want to make sure this is a relationship that keeps everyone involved happy - so I want to give M what he wants.

On the other hand;
- I have never experienced a non-monogamous relationship. I have considered it, but never come to a conclusion on if I want that or not. I feel that I have the communication skills required and could logistically manage it, but emotionally, I'm not sure if I lean any way in particular.
- Like I said, I don't know this friend very well. We've had calls together and talked with M present, but never one-on-one, and I don't really know anything about him. This is a simple fix - just get to know it better - but still a scary solution thanks to extreme anxiety.
- This friend is also online, and lives in another country. I do not like online relationships - I'm only okay with dating M online because he'll be getting here in person soon, and the time until then feels strained because I prefer knowing people in person. If this friend enters our relationship, I'm not sure how long it would work out due to my struggles with online relationships - it feels like it's destined to end, which makes me worry about the impact that could have on me and M's relationship.
- The friend is aro-ace but interested in M. This means the chances of them also liking me are... Low. If I am to enter a polyamorous relationship, I would prefer to be dating everyone involved, otherwise I worry I can get jealous or uncomfortable. I'd be willing to consider M dating them without me, but I'd prefer things the other way.

I just spent about three hours on call with the both of them playing games and I can't really figure out how I feel yet. It's nice, I think, but also. It felt weird, all of us being affectionate towards each other. I really think I just have to be straightforward about all my concerns, but I also don't want to push back too hard on an idea that sounds nice in theory - and I'm not sure what my feelings on this even are.
So, any ideas? Insight? Advice? Poly folks, did you guys know if polyamory would work for you before you tried it, or is it something you have to try in order to know your preference? What's the least risky way to go about this?

[char]

Hey Bugs, welcome to the boards! Personally, I'm in the process of figuring out my preference when it comes to intimate relationship, so I wouldn't say I'm polyam. But as an aromantic person who refuses to be in a "conventional" romantic relationship, I'm definitely open to discussing my relationship structure with my potential partner(s). I'll ask the other staff and volunteers who identify as polyamorous and/or have had more experience in this to help you further!

First off, is M aware of these concerns you have about opening your relationship through past conversations (as well as your preference to not have online relationships), or would this be the first time you're discussing how you feel about polyamory to him? To me, it seems that you have more worries/"cons" than the things you enjoy/"pros" when it comes to this issue. I'm also wondering how things are going between you and M; how do you typically spend time together? If you disagreed to having a polyamorous relationship with M and his friend, how do you think that would affect your relationship, particularly since M is going to live with you soon?