Three: it's just a nice, round number. You have a beginning, middle and end. Good things are also said to come in threes, but according to Lionhead's Peter Molyneux, Fable 3 doesn't necessarily close the book on Albion and the franchise. During an Inside Xbox segment at Eurogamer Expo (transcribed by GamerZines), Molyneux said he hates "limiting" the studio to a trilogy. "Forget about the word 'trilogy.' I'm not going to tell you what happens at the end of Fable 3 but I don't think you'll feel in any way that you've reached the end of a trilogy."
It's by no means confirmation the series will extend past the third entry, sure, but if Fable 2 is any indication, we'll likely feel the same way at the end of Fable 3 as we did two years ago: wanting more regardless.