TC3 is full of British content. Do you want British content only? Most things will not work in Trainz Classics, as TC 1,2 & 3 don't have the usual content, like the PB15 interior and sounds, which a lot of engines need.
If you want a full version of Trainz that most extras will work in, choose either TS2010 or TS2009.
I started with TC3. I loved it! I'm an Aussie who is into British trains only. Although, if you live in America, I assume you will want American trains. If so, TC3 is the completely wrong choice!
TS2010 is a better choice, but if you cannot run that, try TS2009. It is almost the same as TS2010, the only differences being the lack of the Multiplayer patch and no SpeedTrees.