You have to mix all 3, and the concentrations change at each stage of plant growth. Make sure your water pH is in the right range - 6.0 is great all around for veges -(for a quick discussion see hydroponica.blogspot.com) and then mix 1.25 mL per gallon of each
nutrient into the water, stir/shake then add next. You must have all 3, because there is no 1 single nutrient that works! Plants must have several compounds that will not stabalize together....so a 3 part nutrient will always out perform. Send me an e-mail if you have more questions! as the other poster said..use the GH nutrient calculator or it is also on the back of the bottle. 1/4 tsp (1.25 mL) per gallon. You can get a graduated syringe from a pharmacy for free. It will be in mL and uL (microliters). the GH calc will call for 1.5 mL per gallon.