You want to end up with 120 ml of nic at the target strength of 3mg per ml.
So the math says the finished 120 ml bottle will have a total of 360mg of nicotine in it.
120 x 3 = 360 (total mgs in the final mix)
To get 360 mg using 100mg per ml nicotine you will need 3.6 ml.
But to be truely accurate you will need to add that 3.6ml to 116.4 ml of your zero nic e-juice.
In other words, replace 3.6 ml of your zero nic juice with 3.6 ml of your 100mg nic and you will end up with a 120ml bottle of 3mg per ml finished
e-liquid.
Of course as azplumber pointed out, you could just add 4mg and have a 3.225 mg juice. It's really not any significant difference in nic content to hassle with imho. I just wanted to show you the math involved since it's good to keep it in mind when mixing e-juice.
hth
