It is a rather complex process. Firstly, You cannot jsut transfer stuff from SFC2 to SFC3, Too much is diffrent.
When you download a new Ship(From here or one of the Mod Sites) you need to place it in the SFC3/Assets/Models Folder.
Next, You need to add the Spec lines for the ship to the files named defaultcore.txt and defaultloadout.txt which can be found in SFC3/assets/specs folder.
After this, if you so desire, you need to add names for your vessel to the file shipnames.txt in SFC3/Assets/Strings
Now one small warning, a lot of ships wont have their Spec lines, which makes this a whole lot harder, but you can usually ask(Nicely) someone such as myself to write you a spec file for some new ships.
I know this is rather basic, but it should get you started, Im sure someone else will know where the more precise tutorial can be found. best of luck, and Welcome aboard