Hi amnj,
For shortening the length of a piece of foam, there are some instructions for cutting foam in post #3 here and some additional posts about cutting foam with more comments and some pictures in post #19 here and post #1 here.
As for slitting the thickness from 3" to 2", that requires specialized machinery and the only option that would make economic sense would be if you had a foam fabricator or manufacturer close to home who would offer such a service, as the equipment for this (specifically latex) is quite specialized, and shipping to a fabricator who offers such a service would more than likely end up being more expensive, or at least close to the price of a new piece of foam.
Phoenix