Friend help?Another person trying to end our friendship.?

ok we'll say I'm jess, my friend is umbrella annd the girl ending is named duck. so me and umbrella were so close. great friends but as we got closer and closer duck, who is umbrells's best friend and neighbour. she acts like she owns umbrella she always makes umbrella cancel plans with me so she can hang out with duck. duck acts like my friend but its weird. duck makes umbrella dislike me and I don't want to lose my best friend. I mean for example one time; me and umbrella were staying after school for extra credit duck found out and HAD to come. She couldnt stand us being alone for 1 hour after school. I dont want to sound but I want to hang out with my friend! like so i can tell her stuff i cant tell her over the phone and at school. umbrella is my only true friend in my homeroom and i would be really upset if i lost her because of duck. should i tell her how i feel that i feel that she isnt like she used to be? or should i confront duck? i dont know what to do. please help I'm desperate.