Friendship advice

My best friends are closer to one another than they are to me. I feel like they hide things from me and it really bothers me since they have done it before in the past (they secretly dated and didn't tell me until they after they broke up.) I think they are dating again and if they are it really bothers me that they would hide it from me for a second time. I don't want to come off as this jealous super clingy person, but I also don't like feeling left out.
What should I do?