Converting 3D Model formats

I am trying to find a way to edit and or convert .3dp models for the game Railroad Tycoon 3. The only program that I have found that comes close thus far is called 3D Model Converter. I have the free version, which only allows veiwing .3dp models and the full version cocts $50, which isn't happeneing, especially since they won't let you try the program to make sure it will actually convert .3dp to 3ds or some other common format. Does anybody know of a cheap (free)program that will work with .3dp models or convert them for use in a common format? :confused