Please advise if it's okay to mix one percocet 30 and one tylenol (acetaminophen) #3?

If prescribed: but I have a feeling that it isn't prescribed, at least for you. There is no "Percocet 30" because Percocet contains tylenol (acetaminophen) as well, and the 30 mg strength means it is plain oxycodone. There is no reason to mix it with Codeine unless you are going for abuse, i.e. getting high. If you are not tolerant to opioids, 30 mg is a sedating dose, and adding 30 mg of Codeine adds to the sedation. So, NO.