The easiest (and most misleading) way to calculate nic intake would be to multiply the concentration by volume; 3ml of 12mg/ml would be 3*12 = 36mg of nicotine that finds it's way into your atomizer.
But there's the rub: how much of it makes it into the vapor? How much of that vapor enters your body? Of the amount that enters, how much actually has a physiological effect? These are questions that cannot be answered casually, and estimates can vary wildly, from 25% or less, to 70% or more: not very helpful, but it's certainly not 100% of those 36mg.
Even if you did know, could you compare it to anything? I know I couldn't; cigarettes had an unknown amount of (total and physiologically active) nicotine, as far as I'm concerned.
To actually give a useful answer, order some 6mg and 12mg of identical flavors. Depending on the ratio of the two, you can make juice anywhere from 6 to 12mg/ml.
For example, 3 parts 12mg/ml to 1 part 6mg/ml would yield 12(0.75) + 6(0.25) = 10.5 mg/ml, a much milder transition step than straight to 6mg/ml.