Go to UCLA, the better of the three. But if you MUST compromise with those choices--pick UCSD. Less social life, but if you plan to get a 4.0 you wont have one anyways. However as a chemistry major, Berkeley has one of the best chem department I have ever seen!
Berkeley is definitely harder because they have those paranoid premeds that literally memorize everything for the exam, but who doesn't. They eat, sleep, and **** all day in the libraries until there is 5 minutes to the test--UCLA got those too but we occasionally go home and visit Westwood, whereas Berkeley are mostly homeless (literally and metaphorically--no pun intended).
I have many friends from all three of these schools. Most premeds had no regrets with UCSD (can't say the same for UCB), however the non-premeds seemed to enjoy Berkeley FAR more. UCB has more school spirit and events but UCSD is more chill, is far cleaner, and the students are not afraid of stepping outside of their school (again can't stay the same with UCB or USC).
Although, Berkeley is borderline northern California and have one of the best sites for snowboarding, white water rafting and all other outdoor activities. Yet, San Diego is closer to LA and can have both the quiet home town and the metropolitan day trip.
Hope this helps.
*I realize that my writing prob sounds like a third grader, but I am too tired to change it and I just spent the entire day doing MCAT passages and I'm just drained. Thank god we have SDN to feed my mindlessness.