So much misinformation, so little time.
Whether or not you interview at SD has absolutely no bearing on interviewing at SF or LA, so chill out. Like you said, you just got an invite from Penn, so you know you're competitive for the top schools. Be happy with what you have and don't stress over what hasn't even happened yet.
Secondly, I only know of ONE person last year who interviewed at every single UC campus -- they are all pretty different schools and are looking for different things in their applicants. From my experience, UCSD definitely places more emphasis on research experience and interest in academic medicine, while UCSF is definitely more on the touchy-feely side of things and likes people that have done something either very unusual or very outstanding with their lives -- for whatever reason they LOVE teachers -- if you taught for any amount of time, you will get an interview at UCSF.
Anyways, my take-home point is that there is not a lot of rhyme or reason to this process. A lot of it comes down to intangibles like whether schools even have a "niche" for you -- for instance, a basketball team won't recruit all 15 of the best point guards in the nation, because they don't want a team full of point guards. The make-up of a med school class is somewhat analogous.
Relax, and enjoy the opportunities you've recieved so far.
