I notice UT3 only appeared to get a luke warm reception the offical UT2003/04 site....
It sold reasonably well, but the online component flopped big time in terms of activity. There were rarely more than four or five populated servers of each gametype at any time.
There wasn't a lot wrong with the game itself. The weapon balance is a big improvement over 2004 and the engine looks great (IMO second only to Crysis) and runs very smoothly, although the interface feels consolized and has several annoyances. The small gameplay changes they made to each game in the series fragmented the UT community over time though. The release was also poorly timed, coming alongside a lot of other high profile games, and the basic style of gameplay is unforgiving to noobs and has fallen out of fashion these days, so it was never going to attract a lot of newcomers.
The mod community has really saved this game though. The highlight for me is the Galtanor's Invasion mod. Even in singleplayer, it's more fun than most other FPS I've played in the last few years, and it becomes even better with a few friends.