Well, I imagine one can easily derive original gravity of cider from amount of cider, and sugar. Unless different ciders have different amounts of sugar.
Based on this
If we assume cider has 28 grams of sugar per cup. There are 16 cups per gallon so there are 28grams * 16 cups = 448 grams per gallon = .9 lbs per gallon.
6 gallons * 448 grams = 2688 grams = 5.9 lbs of sugar.
Then you added three more pounds of sugar for 8.9 lbs of sugar total.
1 lb of sugar has 46 gallon gravity units. That is, if you add a pound of sugar to 1 gallon of water you will have a specific gravity of 1.046.
8.9 lbs * 46 gallon gravity units per lb / 6 gallons = 409.4 gallon gravity units / 6 gallons = 68 gravity points.
So O.G. = 1.068
*IF* apple cider has 28 grams of sugar per 8 oz.
Why don't you post to the cider forum?