i've used coffee before by racking onto grounds. Worked well, but be cautious on the amount and the duration of time you leave the beer on the coffee. 3oz sounds right for 5gal, i wouldn't go any more unless you like a little beer with your coffee.
Also i would only leave the beer on the grounds for 24 hours, the longer it sits on there the more coffee flavor is extracted. I left a batch on for 72 hours once and it was a mistake. My brown ale was black and the taste was over whelming.
If you do decide to brew the coffee do a cold brew with a french press. If you heat up the coffee you'll extract oils and tannins from the coffee beans.