Queries!

Aak! Eek! Ugh! Okay now that I've got that out of the way...I want to ask a question...At least I think I wanted to ask a question....Wait, let me think about this. I've got all these questions rolling around in my head.  Stop! Okay had to stop the roulette wheel and pick the one I wanted. Okay here's my question... No wait let me give you a little history first then I'll ask.

I am writing a series of 3 books. I am almost finished with the third. I just need to fill in a couple of gaps, tie it together, and make sure it's flows and keeps you reading. But the thing is I haven't written the first 2 books. They are still in my head. When I first started writing the third book I had no plans on writing the first two, because the third book has enough merit to stand on it's own, is a good read, and has the happily ever after, and yet leaves some questions unanswered, that would make the reader wanting more, which then I could write the first two books. I just feel that if any get published, I want it to be the third.

Okay so here's my question for you....

When sending out my query letter, Do I just query the one book as a stand alone? Or do I need to put in the query, that it could be a series?

Is there a difference in tactics when sending to an agent? Or a publisher?

As to that I mean... Do you tell the publisher that you are querying that it could be a series or not?

Do you tell the agent that it could be a series or not?