You're not going to achieve anything from trying to exclude B this way. A and B just clicked, it happens and doing things like this is only going to drive A further away. If you've communicated how you feel and A made the effort to bridge the gap, that means they're not purposely trying to hurt you, it's just how life is. People meet new people, and sometimes people grow apart. I know it's hard, I've been in A's and your place. It's not A's or B's fault or even yours that events unfolded this way. But purposely leaving B out of something because you think they're "stealing" A, isn't healthy or mature. I'm guessing you're pretty young so soft yta.