Best friend trouble

I have this friend, he’s always been really upbeat and willing to help me, and we’ve always been there for each other. Recently, one of my other super close friends started hanging out with him, and I often feel like I’m a third wheel. They always hang out, and he’s starting to develop alot of toxic tendencies I’ve seen in my other friend. He swears that he cares about how I’m doing, since my mental health has been in a rough spot, but he never texts or calls first and I’m starting to get really worried since he briefly mentioned he’s having a difficult time with his own mental health. I tried talking to him about it, but he is always avoidant and I’m very unsure about what I should do.