That 3 gray certain death would assume no medical treatment. Hospital low-tech of IV fluids and transfusions can quite easily manage the lethal effects and make a 300 Rad exposure (I despise SI units) quite survivable.
Yes.
Immediate bone marrow transplant would be the best treatment.
But, up to 500 rads and above, GI syndrome kicks in and there is no effective treatment for that, as far as I know.