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I have seen postdoc job adverts which were asking for as many as three recommendation letters, despite being supervised positions and inviting freshly graduated PhD students. Is this practice common and reasonable?
As a more practical question, how should an applicant deal with this situation?
It seems to me that it is not unusual that the only person a student worked with extensively is their advisor. In fact it would even seem like a good thing to have an advisor who does not lead a large group and has time to work closely with all their students. Yet such students would find it rather difficult to obtain three recommendation letters.