I think your firts two candidates (your boss and HR manager) are perfect. Don't worry that they're both from your company. As long as they can write about different aspects of your profile (i.e., they won't write the same reco with the same examples), that will be fine. Schools want recos from people who know you best, and if they happen to be from the same company, so be it.
For the 3rd recommender, once again you should think about who knows you best and can write a reco that will complement the first two recos. All things being perfectly equal (which is neer the case!), I'd pick the person whom you know outside of work. But most of all, go with the person who knows you best.
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