It's not quite correct. There's a tiny increase in production from having a second farmer; none, as far as I can determine, from having a third.
It is, however, extremely inefficient to use more than one farmer. Only one actually gathers at a time, so the only time you benefit from the second guy is while the other guy is walking to the granary/town center.
You can test it on the Scenario Editor. I think I read it first on the Shai'tan page, though.