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All three.

Stressed /ðɛm/ — often used for emphasis or to distinguish use from mention.

Unstressed /ðəm/ — used in most situations in free variation with 'em /əm/.

(Note: 'em etymologically derives from MidEN hem, not them, but is today often treated as a form of them.)

I feel like I use 'em more often in casual speech and them in more formal or careful speech, though both forms might appear in either register in varying proportions. The rules of when I might favor the one or the other are inconsistent, I feel, though I think it has a bit to do with clarity (i.e. avoiding 'em instead of them, when 'em might be mistaken for him); and ease of articulation.