Assuming you are talking about a quake3 pk3
A .pk3 is a .zip file. Rename it first to .zip then extract it. The important file extensions you should look for are:
*.md3 <-- That's the model data right there. Look for a converter (or use milkshape) to convert it to .3ds or .x
*.jpg, *.tga <-- texture files. Since quake3 supports multitexturing and shaders, it would be a pain if the model requires more than one texture on the same skin for the model. Confusing if you don't get it, but usually you will have to re-texture or edit the texture of the model because of the following problem:
*.shader <-- If you see one of these, prepare to pull your hairs out. It means that the model's textures probably use some programmed shaders, so the effect in quake3 is better (prettier, and more correct) than when you convert it and texture it with some base texture.
"Story in a game is like story in a porn movie. It's expected to be there, but it's not that important." - John Carmack
