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First of all, none of the people you mention is a graduate student. They are all post-docs.
A professor (not necessarily a full professor; an associate or assistant is fine) is always the best option. It's better to use a professor who had you only in class than to use a post-doc. However, if a post-doc is all you have, then you have no choice -- go for the one who knows you best.
When it comes to LORs, most applicants have a weaker third letter, simply because the nature of research makes it difficult for a student to have the time to cultivate deep relationships with three professors. Although those who have three strong letters stand out, those with two strong ones should get good results as well. That said, the third letter needs to be from an appropriate source. For instance, a friend or an employer in a different field will not do. The writer must be in a position to discuss your abilities as a scholar.
Obviously, an LOR from a postdoc is better than no third letter. However, you need to think seriously if you have another choice. Because of your switch, I would think that an ecology prof would be an option, especially if he/she can speak to your abilities as a student/researcher. Who wrote your LORs to get into the MSc program? Maybe you can ask one of them.
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