I am just jumping into the TwentyEleven bandwagon, and I am a little puzzled by the apparent absence of a 3-columns template file... It is advertised in the theme description though. Am I missing something?
It doesn't have one and it doesn't advertise it either. Check the WordPress.org page, it advertises "three layout choices" and "three footer areas" but nowhere does it say it has a 3 column design. In fact, on default single pages it doesn't even have a 2 column design as it has no sidebar.