mixing 3 5gal batches and dry hopping

Hey guys so im brewing a pale ale for a wedding im currently brewing 5 gallons at a time all grain. So on a Saturday I brewed 3 5 gallon batches of the same recipe which was a 12 hour brew day! I want to mix all of it together before bottling to ensure consistently throughout. What would be the best way to go about this?
A. Dry hop individually then combine
B. Combine all 15 gallons, then dry hop
Also if im dry hopping 2oz per five gallons, would I use 6oz for the 15 gallons or does that number change. Sorry if these are dumb questions im not used to brewing on a larger scale. Thanks for your help

this is what im mixing the beer in dont know if that helps http://morebeer.com/products/speidel-plastic-fermenter-60l-159-gal.html

I would say combine them and ferment with an appropriately sized starter. As far as dry hopping, I'm not sure. I would think you'd definitely want all six ounces for that killer aroma.

thing is i have all ready fermented all 3 seprate batches. i kind of want to mix them together in a "secondary" for dry hopping, or should i dry hop before mixing?

I would definitely combine, then take a sample to see how much dry hop you feel is necessary.