State your case to all three of them. Don't be a wimp about it - be positive. Tell them as far as you know, you haven't plagiarised the work, and that if your teacher can't state or show what she maintains is the original, then she'll have to accept your word. There's also the fact that she's teaching the same thing to, what, twenty or thirty odd other students at any one time, all with access to the same sources of information (probably recommended by her anyway). Loads of teachers complain that one student's essay is much the same as another when this is the case. If you know in your own mind that the work is yours, then that's all that matters.
At the end of the day, she's only a teacher, and probably couldn't do much better herself (they don't always know more than you do, you know). Those photocopies worksheets they give you in class? They don't make them themselves, you know. They share them around at all those conferences and courses they go on, or lift them from books and/or the Times Educational Supplement - I know, because I've often had to prepare them for teacher training and development courses.