Inadequacy with friends... :/?

I have two best friends, X and Y. X is a girl, and has been an extremely close friend of mine for 11 years. Y is a boy, and I have only known him for 4 years, during which time we have had a mive fight and made up again, and we have recently become great friends over the past year. Lately, it's dawned on me that X and Y are VERY similar (not in a romantic context). They've been talking a lot together, and not necessarily involving me in stuff they want to do. And recently, X has not been talking to me much at all. When I asked them about this, they simply said nothing was wrong, and that of course I was very important to both of them. Then they just kept doing what they were doing. I'm feeling really inadequate around them, even though they are supposed to be my best friends. I also care about them both A LOT (as friends), but I don't know if the feeling is mutual. Any input?