Hi! (A Practical English Grammar - Thomson, Marinet): An action which began in the past and is still continuing or has only just finished can, with certain verbs, be expressed by either the present perfect simple or the present perfect continuous. Verbs which can be used in this way include: expect, hope, learn, lie, live, look, rain, sleep, sit, snow, stand, stay, study, teach, wait, want, work.
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It is acceptable, but not as precise as the alternative "has been watching".
But is it correct to describe this situation by saying,
<>He has watched TV for three hours