It looks like the semi-colon is throwing you off. You heat sugar in a double boiler, and we are telling you that for 4 cups of juice you will need 3 cups of sugar. So you just have sugar in the double boiler, 3 cups, nothing else. You are warming it up. When the juice is at a full rolling boil, add the warm sugar.
Hope that helps!
It looks like the semi-colon is throwing you off. You heat sugar in a double boiler, and we are telling you that for 4 cups of juice you will need 3 cups of sugar. So you just have sugar in the double boiler, 3 cups, nothing else. You are warming it up. When the juice is at a full rolling boil, add the warm sugar.
Hope that helps!