Short Answer: NOTHING!
In all seriousness, since you're 18, I assume you're in high school. Let me tell you something I learned; high school relationships are idiotic. I just came out of an exhaustively long relationship, and I have nothing to show for it. The girl was incredibly overbearing, and continues to be, though I have made it clear that I do not wish her to be.
High school girls are incredibly idealistic in their view of relationships; everything is a fairy tale to them. This girl is attempting to live out a fairy tale; if you reciprocate by giving her a gift for something as stupid as a three month anniversary, it will only go downhill. I guarantee it.
Whle this may seem cynical, I speak from my personal experience. If she becomes hurt because you didn't give her anything, then she is not a person worth dating long term. In fact, I would recommend forgetting the whole LTR thing for now; have fun, mess around. You don't wanna be stuck.