I really like it when I start a new trilogy and I know all three books are released. It is annoying to start a series and then to realize the author never finished it.
For print - I prefer three books as they are easier to hold.
For ebooks - I prefer one ebook (clearly marked that it includes the whole trilogy) so I don't have to go looking for the other two.
And I really like it when the first book in the trilogy is released free...
