I've seen a couple of new authors get published with trilogies: Russell Kirkpatrick and Peter V Brett being the ones in my particular collection. Perhaps fantasy bends the rules a little more?
More likely, the first book each published was sold to the publisher as a standalone, and was successful enough that the author was able to sell the remaining two pieces.
fantasy does often bend some of the rules... but added to that, Russell Kirkpatrick, a new zealander, was well known there in other fields and was first published in new zealand, which can explain his exception to the rule... and only Peter V Brett's first book of that trilogy has been printed so far, the second not due till next year... plus, he'd written 3 other novels before the trilogy...
