POKASEEDES:
Good morning to you. This recipe that you have altered is known as a "HI RATIO CAKE FORMULA". Why??? ... whenever the weight of the sugar exceeds the weight of the flour you need to either use a ingredient known as "EMULSIFIED SHORTENING"..... or alter the the recipe.... or change the mixing method.... or both to the altertering & mixing method. Emulsified shortening is what comm. bakeries employ for this type of recipe. You were right about 3 cups of sugar being excessive. 3, cups weighs 21, oz. 2 3/4 cups weigh approx 19, oz.. The flour weighs in at approx 13,oz. but the sugar it still somewhat excessive. Also 325 degrees is too low. this type of cake requires 350 degrees unless you add honey then 330 335 degrees is perfect. Of course if using a convection oven than 325 degrees is perfect. Your recipe requires at minimum 1-1/2 tsp of baking powder along with the soda. I would consider 2 tsp. of powder. I do not think you should be using cake flour unless you increase the amount of flour somewhat. The 2 sticks of butter is more than enough of fat, omit the shortening. One more thing, Look in your baking books for the special mixing method that you should be using. It is called the "TWO STAGE METHOD". If you need more help post back.
Good luck to you & have a nice day.

~CHEF DUNASK.