So I've been outlining a story I hope to turn into a trilogy. The 3 books will have their own plots, but at the same time they all work to solve the main plot that extends all three books. My problem is I have an outline for the first and second book, but when it comes to figuring out what will happen in the last book, my mind comes to a blank. I know that book is basically just solving the problem that existed through the whole series but I wish I could come up with a unique plot that would tie into solving the major plot of the series (if that makes sense). The only things I can think of are my characters finding some object that will then destroy the antagonists goals. But this is somewhat of what happens in my first book. I just can't think of anything new....arggg....

So for anyone who has written or is in the process of developing a trilogy, how do you come up with a unique plot for each book? Does it just take time? Any tips would be greatly appreciated.