I took 5ml of hydrocodone cough syrup three hours ago is it safe for me to take a Benadryl pink pill?

Mostly. There is likely some anti-histamine in the cough syrup. That and not the narcotic is probably the bigger risk. And that isn't a big risk. Furthermore, the peak effect of the hydrocodone was by 3 hrs and by now it is wearing off. But if there was anti-histamine in the cough syrup it is still there (but not at peak). These are safe with a wide range of safe doses.