Keeping in mind that I'm not a Bio major:
Caltech only offers Biology as an undergraduate option (the words option and major are somewhat interchangeable here). It gives you an overview of different areas in biology, and then you can use your biology electives to take courses geared toward whichever area you're most interested in. There are graduate options in all three areas you mentioned, so there is defintely a faculty presence for each of these areas, and there are undergraduate courses at least for biochem and molecular bio, though I'm not sure about biophys. I'd suggest you check out the course catalog, which will list all the requirements for the biology option and will list all of the classes offered. You should also look at the course schedule, which will give you an idea of how many courses are offered at a time (since we're a small school, courses in some options are taught every other year -- I'm not sure how prevalent this is in the bio department).
I do know that the biology department is phenomenal here. It's one of the most common majors (if not the most common major), our president (David Baltimore) is a biologist (nobel-prize winning), and we have lots of new state of the art facilities. Remember we're a research school, so the education here is geared towards preparing biologists for research careers, not med school. That said, I do have friends here who plan on applying to med school.
The biggest attraction for undergrads in the biology department is the research opportunities. There are many many opportunities in all different fields of bio, and they're relatively easy to get. Check out the SURF (Summer Undergraduate Research Fellowship) program, through which a lot of students end up with research grants every summer. You can get a SURF as early as you like (they aren't reserved for upperclassmen) and you can repeat as many times as you want. Basically, you find a prof who has research in a field you're interested in, you ask him if he has ideas for a SURF project, you ask him if he'd like to be your mentor, he helps you write a formal proposal for the SURF, then (if you get it) you get $5000 to do a 10 week research project supervised by the prof (or someone in his lab). The numbers for how many people apply and are accepted should be on the website.
Biology Webpage: http://www.biology.caltech.edu/
Course Catalog: http://pr.caltech.edu/catalog/
Course Schedule: http://www.registrar.caltech.edu/
SURF Webpage: http://www.surf.caltech.edu/