The differences between all three schools is pretty minor. UCI, UCSD and UCD all have good econ departments.
However, UCI will be offering a BA in Business, starting in 2008. http://www.redorbit.com/news/educati...008/index.html
As of now, Berkeley and Riverside are the only UC's that offer a BA in business. Because of high demand for this at UCI, I would expect competition to be fierce.