MY FRIEND IS STEALING MY BEST FRIEND?

Al together, my friends and I are a group of four. Theres me, my best friend "Tina" and "mandy" and "joslyn" who are best friends. Recently, "Tina" and "mandy" have been ditching me and "joslyn" and now I feel, a bit ridiculously, that "mandy' is stealing my bestie. I think "joslyn" feels the same way and we're close too. even the other ppl in my cl noticed! I don't know what to say to both of them! Oh, and just yesterday, "Tina" and "mandy" totally ditched me and went somewhere together-and get this. It was supposed to be just me and "Tina" then "mandy" just jumped on the band wagon and once again, went somewhere with "Tina". What should I do?