Well, this scenario is pretty common.
At this point you really have two choices.
1> Continue to stay at the "fringe" and accept whatever conditions are in place that keeps them in their comfort zone and see how it evolves over time. Things may - and DO change. And sometimes they don't. So it's a risk of your time & emotions. Like a loan that may or may not be repaid. But given time for her to adjust, if she comes to the realization that the relationship between them and the love does NOT change because of your presence, things could grow.
Remember, the vast majority of people drifting into poly relationships have NO experience and a lifetime of reprogramming to overcome. It can't - and doesn't - happen overnight. For some it seems too big a mountain to scale. You don't know unless you sincerely try.
2> Back gently out, explaining to them that you care for them too much to inject pain & drama in their life. Let them know that if they do some work and feel better prepared to explore, that you'll be willing. But time may change that too. But try to make the exit in a kind & loving manner. There's no harm, no foul here. Not everyone is up to the task. This is probably one of the hardest things about being a poly loving person in the current society. You are the exception rather than the rule, which means that often there's more clouds than sunshine. But that sunshine is so warm and beautiful when we find it, we soon forget about the cloudy days.
Good luck !