Hey! I'm newly in a triad with a couple that's been together for almost 5 years. I don't really know how to tell if I'm in love or just really like them. All the previous definitions of love I've had were very monogamous and idk how to draw the line between just really liking them and being in love.
This is a tough spot to be in - we are taught through media and socialization and all sorts of other sources that you "know" you're "in love" when you "only want to be with them" or "see yourself with just them for the rest of your life." So it's very fair to feel adrift and confused when you're in a romantic situation that you don't have a lot of models or blueprints for.
My advice would be to let go of this question. There is an entire dynamic, complex, highly individualized realm of human emotion that can't really be distilled down into a binary of "in love" vs. "just really like." You feel what you feel right now - find words for it that fit, without worrying about whether a specific word tied to a limited construct fits perfectly.
Do you feel committed? How committed? What kind of sacrifices and compromises do you feel willing to make? Not willing to make? What kinds of songs, images, and symbols capture your relationship well? What do you like to do together? What makes you feel happy, grateful, or fulfilled in this relationship? What positive things about yourself do your partners draw out?
Find ways to describe and understand your relationship that are unique and specific - because your relationship is unique and specific. Everyone's is, whether they're poly or mono. What I feel as "love" for my partner might not be what someone else identifies as "love." What I experience in a healthy relationship is different than what someone else needs. Don't worry about whether what you feel meets the "true" or "real" or "correct" definition of one word.