I took benadryl around 9pm. Can I take nyquil now at 3 am? Unable to sleep due to congestion/post nasal drip/thick mucus.

Yes. However you are not like to reduce your nasal congestion from these medications. If this is from a cold, an OTC decongestant spray for no more than 5 days may be helpful. If this is from an allergy , try a nasal steroid. If problem persists, will need to consider the presence of a sinus infection and you will need to see your doctor to get examined.