I think you should tell her that she can go - if she earns the $ to go. (Depending on the cost of the trip, you might say that you'll match her, if she earns half, you'll cover the other half).
I know she's not old enough to get a real job. But she could babysit, mow grass, pull weeds, be a dog walker, etc, for your neighbors. I had a neighbor boy who did this exact thing (was earning $ for a trip), and thought of something for every season: at Christmas, he put out a flier advertisting gift wrapping services, at Valentines' Day, he made and delivered heart shaped sugar cookies (again with fliers with forms for people to order ahead of time), plus he did yardwork. You could also assign monetary values to things you need to have done, but I think it will make more of an impression if she has to earn the $ from someone other than you.
And since it's likely that she won't be able to raise the funds to go on all 3 trips, she'll have to choose one of the trips to focus on.
She is beyond old enough to understand that money isn't free and that sometimes you have to make choices because you can't afford everything you want. And if she isn't willing to work for it, she obviously doesn't want it that badly - and not going becomes HER choice, not a decision you made for her.
And, all this said, if you believe she is going through a stage where she feels entitled, then a mission trip to see first-hand how lucky she is may be just what she needs at this point in her life.
(In the meantime, you should also find out who goes with the kids and how they make sure the kids don't get into trouble.)