If you purchased UT3 at retail, you can type in your product key on the "Activate a Steam Product" feature in the steam client. Steam will update UT3 for free.
Sadly thats a crock, what it actually does it ignore your existing install and start downloading many GB's so you can have a second install from scratch. Those of us on bandwidth capped ADSL will wince downloading over 5GB of data again.
... UT99 is where it's at