I m buying a 3,000 sq ft home how big of an a/c unit does it need?

I would suggest that you use individual AC instead of centralized AC. It will save you a lot of money in electric bill. Except you are rich enough to pay the electric bill, you needed a 90,000BTU AC, which converted to wattage will be about 12,000 watts.

With the house lay-outs, decide which room should be air conditioned. Calculated the cubic volume, and convert it to wattage using this equations; every 1,000 cubic feet needs about 10,000 BTU capacity AC which is equal to 1,200 watts or closely to 1.5 horse power of air conditions. If you prefer a slow temperature drop, 5,000 BTU can be used as reference.

Hope the above equations will help you but I recommend that you consult your local AC installer as it depended to local situation.