Someone’s stealing my best friend...

I have three main friends, one of which I’m significantly closer to than the others. Let’s call her A. Now, B is a newer friend, but I feel like she’s unintentionally stealing A away from me... B is really nice, so I know she’s not doing it on purpose, but they’re spending so much time together recently, and A has made it clear that she’d rather spend time with B than me. Even C is spending a ton of time with B... she’s unintentionally fracturing my relationships and I don’t really know what to do. I’m friends with her, but not nearly as close as she and A or she and C have become. The three of them do stuff all the time and leave me out, and that really hurts. Except I can’t talk to B about it because that would make her feel terribly guilty and I don’t want to do that to her...