Anthropology major applying to Biology PhD programs... last LOR?

[mo mo]

I'm currently the technician at a lab in an ivy league university, and I'm applying to PhD programs at non-ivies. I know my PI (boss) will write me a good letter of recommendation, and I have a biology professor from senior year of college who will also write me a great one- now I have to decide who to ask for the third one.

I was going to ask another supervisor from a different lab I had worked at, (although it's been at least 3 years that I've worked with them closely) mostly because they will be able to speak to my work ethic and character traits.

However, my anthropology professors know me much better from an academic perspective, and I have old papers I can send them to jog their memories of me. I'm hesitant to ask them though because anthropology is only peripherally relevant to research in the life sciences.

Lastly, I have co-workers who are post-docs who I work with in lab who are very smart and know me very well (both interpersonally and in terms of my work ethic). I'm starting to think that my best bet would be to ask one of my post-doc co-workers (she went to Princeton for her PhD), because the letter would definitely be glowing, but in terms of "credentials", she hasn't had her own lab or anything like that, so I'm not sure what to do!

Any advice would be much appreciated. Thanks!