The following refers to the PS2 version of the game.
The models (among other items), are contained in the .ONE files.
These files can be extracted using the program GameExtractor: www.watto.org/extract/download.html‎
Then the extracted files can be decompressed using the following program: http://segaretro.org/images/c/cc/Prsdec_auto.7z


ShadowHedgehogBuilding.jpg

Some models are in the .RP2 files
Use the script above to convert the .RP2 files into .3ds files with UV coords.

Format of the .RP2 files:
.....
00 00 00 05 04 01 00 01 00 80 XX 68
{4ByteFloatingPointVertexes(X,Y,Z)}
......
00 00 00 05 04 01 00 01 01 80 XX 64
{4ByteFloatingPointUVMappings(U,V)}
.......
Where XX = the Number of Vertexes in a particular section