What is the average time a blood clot would show up after a small hospital stay 3 days with pretty moderate activity. ?

Varies: Overall, the risk of such an occurrence would be low, especially if you were up and about. If you were to develop a dvt, i'd expect it within 7-10 days. First presentation would most likely be a unilateral swollen calf, perhaps associated with fullness or pressure. Good luck.

Blood Clots : Without any details about your age, weight, health status, medication history, reason you were hospitalized, etc., i would be shooting in the dark. Have you had a clot before? Did you have surgery? Do you have any risk factors for a clot ( on ocps, clotting disorder, cancer, overweight)? Typical dvts form in the veins behind the knee or calf, causing redness and local pain. Call you md if concern.