I’m jealous of a poly relationship. I wish I was a part of it.
I’m not 100% clear from your wording whether there’s a specific polyamorous relationship you want to be part of, or whether you just want to be in a polyamorous relationship.
If there are people in your life who you want to be in a relationship with, it’s okay to pursue that! Flirt with them, invite them out, identify what you have in common. Ask them out!
If they make (or have made) it clear that you’re not interested, it might be healthy for you to take some distance from a situation that makes you feel jealous or left out. Hang out with them less, find other friendships and hobbies to invest in, unfollow them on social media.
If you’re interested in dating polyamorously even if you don’t get to date specific people, check out my page on finding polyamorous people to date!