No, they pick out the best parts of all 3 letters. The committee's LOR requirement is that 1 must be from a science faculty (in my case, a BME faculty). The other 2 can be from anyone. Sorry if I was unclear.
Ohhhh ok that makes sense. So you have an engineering letter, a psych letter, and a BME letter and the first one is your science letter, the second your non-science, and the third is the one you have a question about? There are two ways to go about this I guess:
I have seen schools that say a committee letter MUST include information from two science letters (although rare). So, you could play it safe and get a letter from a professor who taught you in a class marked as BCMP on your AMCAS app. While most of the schools seem pretty flexible when it comes to the non-science letter they are pretty strict on the science letter.
Or, you could call schools and ask if they will accept a BME letter as a substitute for a science letter. However, I would first ask if the requirement for the individual letters even applies to the committee letters. Like I said, there were two or three schools that had actual requirements for what made up the committee letter, but most didn't so it might not even matter for these schools (my school had no committee so I'm not too sure about this). They probably will accept a BME letter as a substitute since AMCAS happily accepted all of my BME classes as BCMP, so you might get an exception even if you didn't mark your BME classes as BCMP on your app.
If you haven't actually applied yet and this is for next year, I will say that most likely your BME classes will get accepted as BCMP far more easily than your engineering class (I'm assuming the engineering class you're talking about is an EECS, design elective, or Gen Eng class?). My BME classes were accepted, but my circuit design/analysis classes were not, so keep this in mind as well.
