A doctor had to use her hand to remove my wife's placenta as well, but during a routine exam situation, I can't see a reason they use more than 2.
During play exams, I sometimes examine my patient's vagina using one finger during the external exam, right after I a palpate her Bartholin's glands. I like being able to view the introital area while I assess pelvic muscle tone, and I usually ask the patient to contract/bear down as well as cough while I conduct my visual inspection. I find that one finger allows me to better palpate the tissues of the vulva and vagina.
I have thought of very subtle "bad doctor" scenarios to use with a patient who has consented in advance. For example, on a rare occasion when examining my wife, I won't use gloves, or I will try to use 3 fingers for the bimanual. The point is to try and get away with it, and to realisticlly portray what an inappropriate Dr might do. When you look at actual cases of misconduct, it's usually something similar to that, rather than the Dr pulling his pants down during an exam. I remember during the exam without gloves, she kept picking her head up to look, but the drape was over her legs. I snapped some nitrile gloves to make it sound like I was putting them on, but afterwards she commented how realistic the exam seemed. It's interesting that she didn't notice, but I rarely do it, as if she catches me, she will look for it every time.
I will post the results of my 3 finger bimanual when I can....