Hi there.

Uh, who told you that? I think they must have meant that it's fairly easy to convert a three-speed bike to a fixed gear by swapping out the three-speed wheel for a wheel with a track or flip/flop hub.

The only three-speed fixed hub that I'm aware of is the Sturmey-Archer ASC, a museum piece that no one has.

Sheldon Brown has instructions for turning a standard SA three-speed hub to a two-speed fixed, but even he cautions that it's very tricky.

http://www.sheldonbrown.com/awfixed.html