Use the "FORUM RULES, GP3 FAQ And Installation Guide", which is stickied at the top of the Grand Prix 3 thread list. Also check the .txt files in the zips you've downloaded. Most of the downloads have one. They tell you where to copy and paste the folders that are inside the zips or what names to give folders if you need to create them and what files go in them.
I say 'copy and paste', because I always extract the zipped files outside of the GP3 folder, so if any files in the zips want to overwrite a default GP3/GP32000 file, you can make a copy of the original, for convenience.
The best place for downloads is GPG! There's more support on this site than any other.
Otherwise check out Roberto Scandura's site. Just Google his name. Track installation is a simple labour of love when you get the hang of it. Enjoy!