Two Tylenol 3 tablets is the maximum recommended dosage. Because Tylenol 3 contains the opioid codeine, the danger in taking more than the recommended dosage is that it could cause severe respiratory depression, which could lead to death. If two tablets does not help your pain, contact your doctor to see if he can give you something else that might be more effective.
Yes, sciatica can cause the entire back of your legs to ache. I'm afraid I don't know the answers to your other questions about sciatica, but I will bring your question to the attention of Dr. Lasich, who specializes in back problems.
I wish you the best!