Since Sterno didn't actually post it as an answer, I'll do so for the sake of having the question answered.
In truth, it depends on your skill level and play style. I'm sure as you played more of the series you got better at it so that may have also altered your perception of the experience to some degree, making it seem shorter if not easier as well.
While it's not "official length", as Sterno suggests, HowLongToBeat.com is an excellent reference to find out the average length as experienced by other users. Note that results are dependent on the number of users that contributed their results.
According to the referenced data, Uncharted 3's main story mode alone was a marginally shorter experience than Uncharted, but 2-3 hours shorter than Uncharted 2 as an average and as a median for most of the 150-180 players surveyed. It should be noted that the fastest times and completionist times for all 3 games got progressively longer by several minutes so if you were playing as quickly as you could or completing everything available, it would likely be the longest of the 3 if only by a few minutes to an hour.