Just to clarify and sum all this, there is generally (note I said generally) the following requirements
1) A premed committee (the requirements for that are defined by each premed committee independently)
OR
2) Faculty instructors from 2 sciences classes and 1 non-science class you have taken
OR
3) from 2 science faculty and 1 non-science faculty.
Please note the difference in #2 and #3. #2 requires that you have taken a class from them (so a PI may not qualify for this) where as #3 simply requires they be faculty (so a PI would suffice). Also note where AMCAS only uses BCPM (Bio, Chem, Physics, Math) that is not the limits on science letters. All sciences would technically fulfill the requirement while math and engineering may not depending on the school and course. Some biology leaning psych courses may as well fall into this. It is far less likely, indeed unlikely, a school will reject you for not fulfilling the technical definition of the letter. My rule of thumb is a quality strong letter from your math or engineering prof would likely be "acceptable" as science and worth far more than a piddling letter from the chem prof where you 1 student in 300 in a lecture hall
However, please note that each school has their own specific requirements and some places have very specific and detailed additional letter requirements. For nontrads, some schools give alternative options, such as a work supervisor. Generally letters from doctors you have shadowed are not worth much. My rule of thumb is to get 4-6 letters and have them on file with AMCAS where you can assign them to each school independently as needed
