I suggest that you don't get too tied to doing a trilogy. If you want a trilogy, plot it out and write a synopsis for each book. Then write the first book. Stop there. Try to sell the first book. If no one likes the first book, you haven't wasted your time writing the other two! Even if the third book in the trilogy is the best work in the world, and would rival classics in its brilliance, no one is going to read the first two books to get to the one brilliant part.

I always suggest to new/unpublished writers that they start with a standalone novel that could be expanded into a trilogy or series or group of connected novels.