I definitely recommend beer gas over 100% N2, stays in solution so much better. I have n2 right now and I have to drink the coffee pretty quickly to keep it creamy.

I use these bags: https://www.amazon.com/gp/product/B00M5RQMUS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I grind up coffee beans until the bag is 1/2 full on a 1.5gal keg, so I'd say just fill the bag and you're good for 3 gal. Sorry no weights, I just BS it in my kitchen. If I had to estimate, probably 1lb ish.

I stuff the bag into the keg (hardest part), add water and hook up the gas/serve lines.

Wait 2 days and boom ready to go, might be a little longer since your keg is 2x as big.

I give the keg a little shake right before serving to make sure everythings mixed up, but you don't need to brew in a bucket and transfer after brewing, waste of time unless you want to conserve the space in your keg.