I wrote my first book, totally standalone, but with a door open for a sequel. Had no idea what the sequel would be about, but left a way to do it should it come about.
That was published, and it's gotten good reception, so I thought about it and figured out a nice plotline for the sequel. Will it be a trilogy? Nah, probably not.
I also have a series. But that was planned to be a series, not a trilogy. At least five books, possibly seven. I have, running around my head, another idea for books that will probably be a trilogy, but that's because there are going to be three main characters, and I'm thinking that each needs their own book. Actually it may be a quartet, I'm not sure.
Anyway, all this points to the fact that most publishers won't take on a trilogy by an unknown. Not to say that it never ever happens, but it's not as likely as a standalone. You're an unknown quantity; they want to know how you'll sell before they commit to more than one book in the same series.
Of course people do get mulitple book deals. But I wouldn't count on it.
Just write the book and let the rest take care of itself.