A 3' diameter rock is quite heavy. Volume of a sphere is V=4/3 times Pi times the radius cubed. A 3 foot diameter round rock has a volume of about 14 cubic feet. Stone varies quite a bit in weight per cubic foot, but granite or limestone is about 165-170 pounds per. So a 3 footer would most likely tip the scales at about 2400 lbs. the bigger 4 footer would be around 5700 lbs. I think you'll need a bigger tractor 