Beginner need lots of help

[GpsFrontier]

Do you have a link the the particular nutrients you are using/planing, I wasn't aware that Advanced Nutrients had a 3 part nutrient mix. Or are you talking about 3 different nutrients for 3 different crops? Also I am a little confused on your setup. Are you planing 3 separate systems, with one crop in each? What crops are you planing? How many plants each? What is the size (gallons) of the reservoir/s.

how am I supposed to regulate all the different nutrients if I am using 3 premixed nutrient solutions

I'm not sure if you mean each individual element (nutrient) in a pre-mixed nutrient or not, but if so you cant. That is why you change the nutrient solution periodically to bring it back to a balanced solution.