Tried this?
https://support.steampowered.com/kb_article.php?ref=5357-FSQM-0382 It worked for me with Metro 2033.
BlackMageJ: It should still install from the disc, but Steam will automatically download any patches that have been released since the version on the DVD.
Why would you need three DVDs anyway? Install data is compressed, just because the data takes up 10 gigs on your hard drive, it won't take up anywhere near that much on the disc. The download should be smaller than the HDD requirement too, since again it's compressed and then extracted as part of the install process.
Not always. Sometimes you download the files exactly as they're installed, already decompressed. When you install the game on Steam, all it does is write the proper registry values and install any needed DLLs / PhysX / DirectX stuff.
Sometimes the games are already compresssed, too. And don't forget this is the Ultimate Edition, which already included all the DLCs...