First click on the more advanced calculator at the ecigexpress site.
eJuice Calculator 4.0 | EcigExpress Archive Cookbook
You would input your values as follows
where it says what Base nic level click on the values to change : ie. if you bought 70/30 vg/pg change the default pg/vg that says 50% to what you bought
Then where you see Target nic(meaning what you want it to end up being) in your case 3mg and change the pg/vg ratios
input the amount to be made of finished ejuice (amount to be made for recipe) ie. 30ml
input the % for the amount of flavoring you want in the recipe.
You will see the values at the right of the screen change to what you need to use.
Good Luck
This calculator will do recipes only. Look at the DIY thread for more advanced calculations. ie changing a pg/vg ratio, etc.
Ecf member Scubabatdan has a great calculator. His thread in more towards the top of the diy page click on his signature to download the calculator.