Write each book as stand-alones, and query the first like Ferret said.
Aside from that, if you're hard-set on trade publishing, I suggest you write the first book and heavily plot the next couple and leave writing them off until a publisher agrees to pick up those books--it'll be a waste of your time otherwise.
That said, I also suggest you look into just writing a single, good book. Instead of expending all the time and energy on a trilogy, focus on that one book and pack in all your good ideas. Because that book matters.
Doing the above two will give you more time to write separate stories which could get picked up by agents/editors instead of the one your query. e.i. if the agent/editor doesn't like that one story, you can bring them a separate one (that you spent the time you'd be using for a sequel) that they might like.
