I brewed a three gallon batch of coconut wheat beer many years ago and I think I used one pound.
Use the largest non-sweetened chunks you can find, the small shredded pieces are a pain in the neck.
I used one pound of unsweetened shredded coconut. I toasted/roasted in the oven and used a cheese cloth bag I made then put it in a secondary for a week. It was just enough coconut for 5 gallon batch of coconut porter-it had just the right finishing taste. I would think one pound in a three gallon batch would be perfect if you want it to be coconut forward. Otherwise half a pound.