My friends hang out without me all the time?

I'm really close to these 2 girls and we all get along really well. My 2 friends are really close to each other because they were friends previously before we all started hanging out. I knew one of the girls for 2 years and when I met the other girl I realized her and my other friend were best friends! I wasn't prepared to get 2 friends when I met her, but still we all got along well. Whenever I invite one of them over I ALWAYS invite the other. But whenever they hang out they always exclude me! I get upset because the don't even ask me and then when I text them they're like yeah I'm at Lisa's house, what r u doing? They totally exclude me and I feel left out because we very close and if I'm not making the initiative for us to ALL hang out than its just them and then when we all actually hang they will sit there talking about things they have done without me. I dnt no what to do because it's spring break and they're already hanging out and doing stuff! I'm just sitting here like WTF? Could u at least ask me if I'd like to chill with u guys? Please someone tell me what to do! I'm afraid that if I say something they will get mad.