or perhaps it's a good thing.

Anyway, I have recently become better friends with this group of girls. I have known them for awhile, but we started hanging out more recently, so thing are going pretty good. All of them are seniors, except for one junior. For the record, I am a junior. The one junior girl really likes me. Her friends have said it, I know it, and it can be spotted from about a mile away. One of the senior girls likes me a lot too. Finally, I like another one of the seniors, and we are pretty cool. She hasn't shown the same level of interest as the other two, but she's a little bit shyer than the other two to begin with. So, to sum it up, there are three girls who I could possibly get into a relationship with. Normally, that would be great, and I like all of these girls. The problem is, though, that they are all good friends.

I want to try to get something going with the more shy senior girl, but if it doesn't really work, I am concerned the potential for anything to happen with the other two would be gone, since they are all such close friends.

Am I justified in thinking that, or am I simply too worried?

I think it would be fine if you go for the shy senior.
If anything it would make the other girls like you more (jealousy)

Just go for it...you'll regret it if you don't.

yeah dude, you like her most just go for her