I'm a rising senior in college and I'm looking into grad programs in biology. For the Letter of Recommendation requirement I have found that usually 3 are required.
Currently I can only think of 2 mentors who are in the field of study I would like to go into who can write strong letters. Is it bad if the third came from a professor in a completely different field, or as I like to think of as the humanities research that interprets the science research I'm interested in? Thank you!
Or should I ask one of the post-docs in the lab I work with (in addition to the PI of the lab) since they are working in the relevant field?
For study in the US and those places with similar systems (broad based undergraduate studies), the third letter from someone in another field should be fine. Other things in your application should show what you know and the skills you have attained. The LoR should attest to the likelihood of your success from someone who knows you well. It is fine if you have broad interests. It is good if several people recognize it and are willing to say so.
For places with a narrower educational focus, the answer might be different, but not necessarily.