Actually 4pdrs are about 3.10 inches. That is in a 4pdr "range". 4pdrs were around during the ACW, most examples being of state militias. Ames produces a 4 pdr bronze field piece.A 6 pound cannon uses a ball of about 3 5/8” diameter weighting 6 pounds. So a 3” cannon ball would weight more then 4 pounds.
LOL, yup,,,,It might be a cannon ball.
On the other hand, here's a little story about my younger days:
When I was about 9 years old, my friends and I found a "cannon ball" that was about 3" in diameter. It was made out of iron so we were sure we had found a treasure.
We took it to a local place that bought metal things with great expetations of becoming rich only to be told, "That's a ball mill ball. The big mines use them in their mills to crush the rock into little pieces so they can get the metal out of it. I've got about 300 pounds of them out back if you want to buy some more of them."