I'm in coco so I start feeding seedlings from day 1. A slightly lighter mix (same ratio, more diluted) early on and in the last few weeks.
Being dry salts, you have to mix by weight. So you'll need to measure out each part on a scale (individually) and then stir it in . I don't find that to be too much of a PIA. The 3-2-1 refers to the weight that you use per gallon. 3 grams part a, 2 grams part b, 1 gram epsom salt.
I use a modified version of the formula, which adds MKP (monopotassium phosphate) to the mix. I found it on this video from Greengenes Garden, highlighted at the 29:39 mark. The whole video is worth the watch though, if considering Jack's.