http://store.steampowered.com/app/13210/. Free weekend this week and 40% off (total $11.99) if you buy it before March 15th.
If you bought UT3 in like a real store then you can register your cd-key with steam.
If you are going to try it then I would start preloading it as soon as possible. The original download is 8GB and just today they put out a 300MB patch and a 900MB content pack.