My understanding is that having at least one sequel is a virtual pre-requisite to getting a publishing contract in SFF at present. It's undoubtedly different for well-established authors who might want to put out a one-off stand-alone, but for the rest of us it means having to have follow-ups.
Though having said that, I know of one recently published book from a newcomer which was a stand-alone -- at least I've heard nothing of any follow-up, though it's possible he intends to use the characters again (though I'm not sure how). **
Anyway, having plans for a trilogy is unlikely to be a hindrance, kopiteste. But there may well be a difference between writing a book which can have two sequels, and writing one ginormous thing of 450,000 words and having to split it into three parts. Which of those it is easiest to place, all other things such as writing ability being equal, I don't know.
**EDIT: I hadn't heard because I hadn't bothered looking...
Just checked and the book I was thinking of isn't part of a trilogy but the setting, if not the characters, is used in two other books, which is presumably enough.