Jacks 321 refers to a particular formula for using Jacks nutrients. Originally it was 3g-2g-1g of the 3 dry parts per gallon of water, but Jacks has tweaked the formula over the last couple years.
Part A is a 5-12-26 fertilizer that also includes micronutrients. Originally was 3g of this, but I think it’s like 3.6g now.
Part B is 15-0-0 calcium nitrate. Originally 2g of this, but it’s like 2.4g now.
The third part is epsom salt. 1g of this, or like 1.2g now.
Using 3-2-1 gives you about 80% of the EC of using the actual current “321” formula.
There are several threads here on Overgrow about it. One here geared toward hydro growers: Jacks 321 Hydro discussion, questions, tips, etc
One here for those using jacks 321 and promix:
Formula-wise, jacks 321 is similar to athena pro nutrients, but like 1/3 the cost.