Think of it as this:
for every 1g fat, there should also be 2g protein and 3g of carb. It is basically a rough ratio to get your meal into a zone balance.
If you look at a typical 3-block meal, there is 9g fat, 21g protein and 27g carb. This is more exact, but basically gives you the 1-2-3 ratio.
Or for a snack, there is 3g fat, 7g protein, and 9g carb. Again it is more exact, but basically gives you the same ratio.
It is not necessary to be exact on the Zone, so that is why 1-2-3 is "close". The Zone is a range, not an exact spot.
I personally use the block method, which is just another way of getting there. Sometimes I use gram counting, but I really don't use 123 because it still involves me having to do mathematical calculations, and for whatever reason, it is just easier to know the numbers of grams for a meal/snack, and then to just get the numbers to add up (choosing zone favorable ingredients, of course). Maybe other systems work better for others... it doesn't matter which method you chooose, though, because all methods basically will get you in the Zone.
This plan basically balances out the hormone insulin. But, gaining control of insulin will also help you in your attempts to balance out the other hormones. And controlling inflammation is the cornerstone of the Zone!