As you know cool 3 games are coming out really soon and according to official sources (listed below), those games will be fully integrated with Steam:
So it’s 100% sure that their CD-KEYs will be registrable on Steam.
Now just go and buy those in retail stores, it’s way cheaper than on Steam (see the price difference here).
After you get them enter the CD-KEY in Games > Activate product on Steam…
And that’s it! :D