I'm always the person that's third wheeling or just the back up person

( If anythings spelt incorrectly please don't judge) So basically I have 2 best friends were a trio. (For privacy reasons ima not include their names so lets call one plant and other fire) So plant is always closer to fire and the have inside jokes that I don't understand and i'm usually just tagging behind. Sadly it's just so obvious that she likes fire better than me an example is when I say "Hi" she responds like she's super annoyed at me but when fire says "Hi" she's like the most happiest person in the world. Today we had a movie poster and we 3 grouped up plant was hesitant about me joining her and fire because I "complained too much" but in reality I was just confused on what was she doing. When we started plant did the title and we were all three were just chatting. Once she was done she made me draw the picture. While she and fire was just playing around just stopping a few times to see how much id done while she was super loud laughing with fire. But guess what she's the nicest person in the world when fires not around. I don't know what to do I just feel so left out. :(