Unfortunately, the ability to purchase the original DOOM 3 was removed from Steam and other services with the launch of DOOM 3: BFG Edition. You can technically still purchase the original DOOM 3, but that requires ponying up $100 for the complete id Software collection on the service.
It's not something that will change in the next few minutes, but id Software told me it’s working on a fix.
“We do realize that there are fans that would like to purchase the original version of the game, and we’re looking into options for them,” said the company in a statement. “When there’s new info to share, we’ll let everyone know.”
If you already own the original DOOM 3, you can still access that in your Steam library.
One of the primary reasons you’d want the original DOOM 3 is because it’s moddable, whereas DOOM 3: BFG Edition is not. That will be an option when the DOOM 3: BFG Edition source code is released “in the near future.”