What's the difference between 100mg/ml and the 12mg/ml? I was going to get the 12 because it was closest to 3mg.
Put into words, liquid with a nicotine concentration of 100 mg/ml has 100 milligrams (a measure of mass) of nicotine in each milliliter (a measure of volume) of liquid. 12 mg/ml liquid has 12 milligrams of nic in each milliliter of liquid.
For your OP question, the difference is that 100 mg/ml is about 8 times stronger than 12 mg/ml, so you would need much less of it to increase the nic level of a given juice (thereby diluting your flavor less). It is also a fair bit more dangerous than 12 mg/ml, so read the stickies about safety if you go that route.
In my opinion, regardless of the route you choose, I recommend you read more of this subforum or other resources to understand the basic chemistry involved. You do need to have a good understanding of the concept of concentration and what the terms mass, volume, dilution, and solution mean in this context in order to mix juices safely. The calculators will do the math for you, but just punching in numbers without understanding what they mean can get you in trouble.
[edit: I hope that does not coma across as snarky or discouraging to you, it is not meant that way. If you have specific questions, do ask them here and we will do what we can to help.]