I'm not sure what you mean by only being able to get parts 2 and 3, but I believe that if you install Steam and create an account then you should be able to play Black Mesa without owning any games. What happens is when you install Black Mesa it appears in your Steam list of games and then when you try and run it for the first time (via Steam) it will download the Source engine, which is required to run the game.
You can set Steam not to start at login etc. so it doesn't get in the way, and every now and then they have sales where you can pick up some bargains. Here's the link to install Steam: http://store.steampowered.com/about/
By the time you've done all that you will be pushing 10GB of downloads (both Black Mesa and the Source engine are about 4.5GB), but it will be worth it!