To transfer from one UC to another UC, you must apply in November of your sophomore year. If you apply in November of your junior year, you would probably exceed the unit cap by the time you actually transfer (most UCs do not allow students who are in senior standing to transfer).

According to the University of California:

Intercampus Transfers

After you enroll at a UC campus, it may be possible for you to transfer to another UC campus. Applications for intercampus transfer are considered in light of the applicant’s personal circumstances and the availability of space in the prospective major. Students who want to transfer from one UC campus to another must submit an application for undergraduate admission during the appropriate filing period. Some campuses do not accept intercampus transfers who have completed more than 120 quarter (80 semester) units. Students who satisfy the lower division general education/breadth requirements at a UC campus prior to transferring to another UC campus are considered to have satisfied the lower division general education breadth requirements at the UC campus to which they transfer. At UCR, this option is not available to students transferring to The Bourns College of Engineering. At UCSD, transfer reciprocity is available to students transferring to Warren, Marshall, Muir and Sixth colleges. Students who select Revelle or Eleanor Roosevelt college must satisfy the general education requirements of that college.

Source: http://www.universityofcalifornia.edu/admissions/undergrad_adm/paths_to_adm/transfer/tr_inter_camp.html

You can transfer as a junior, but if you are starting your junior year already, it is probably too late to transfer for this quarter or semester, and therefore it may be too late to transfer overall. The other problem is that you don’t say where you are now, but my guess from the way you say this is that you are looking at going from a less-competitive school to a more-competitive one, which is far less likely to be successful than doing it the other way around. If you are currently at Berkeley and doing well, then yes, but if you are currently at Riverside or Merced, you probably won’t be able to transfer to the schools you mention.

Sure it’s possible. My friend transfered from UCD to Cal his 3rd year of undergrad. I’m not sure if the date to apply for a transfer has passed. Perhaps you should call and talk to someone at UCD/SD/LA. Good luck!

The date to transfer to another school is over, however, they let you transfer still, but it depends on your case. It would not hurt for you to call up and ask. If you can not transfer, try the next quarter or semester.