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?