The BeoLab 3 is the better speaker.
That being said, I still have people in my showroom who prefer the sound of the BeoLab 8000. Most, however, are making their first "high end" loudspeaker purchase. I have a theory that the BeoLab 8000s sound like a bad speaker all-grown-up. They're especially disappointing in the low end when compared to BeoLab 3s (or any ALT speaker), and their sound stage is fragile if present at all.
BeoLab 3s tend to be bought by the more studied client who's maybe had some experience and owned a serious system or two. The BeoLab3 is, in my opinion, one of the best values out there and certainly the best value for money speaker-wise in our lineup.
There is scarcely anything in this world that some man cannot make a little worse and sell a little more cheaply. The person who buys on price alone is this man's lawful prey. - John Ruskin