If I understand it correctly I believe the first addition is bittering, the second is flavour and the
third is aroma.
Is that right?
Yes, that is 100 % right.
The first hop addition to be added is where you start timing your boil from. So if you say want to use 10 gr of Saaz for bittering, you will put 10 gr of hops in say a small netting bag from a $2 store, tie it up and throw it in the pot when boiling. This is your starting point, it decides how long the entire boil will last.
If the recipe says "10 gr of Saaz at 60 minutes" and you throw the hops in at 1:00 PM, you will finish boiling at 2:00 PM.
Then the recipe says "10 grams of Cascade at 20 minutes" and since you plan to stop boiling at 2:00, 20 minutes before that is 1:40 PM, you throw the second hop bag in at 1:40.
Then the recipe says for instance "20 grams of Saaz at 5 minutes", so 5 minutes before end-of-boil is 1:55 PM, you throw in the third hop bag at 5 minutes before you plan to stop boiling.
Hop bag 1: 60 minutes boil, most of the flavor and aroma will boil away, leaving a bitter effect
Hop bag 2: 20 minutes boil, will leave a little bittering, mostly flavour but some aroma will still be left
hop bag 3: 5 minutes boil, not enough time to bitter the beer much, not enough time to give lots of flavor, but enough to give that nice, fresh aroma or smell.
Please look at this graph as it gives a good illustration of
-how long do you boil the hops based on what you are aiming for
-do you need to boil a long time, or are you only after a bit of flavor
If you are doing kit beers, buy a "teabag" of hops, one of those foil bags with 12-15 gr of hops. Don't worry about the AA% as you will not use it for bittering.
Boil it in some water, steep it like a teabag or even boil som Malt Extract and then boil the hops in this, looking at the graph to decide how long to boil it. Aim for say 5-10 minutes as a first go at adding hops.
The shorter the boil, the more "fresh" the beer will smell/taste and you don't want (I assume) to do hop additions to make your beer more bitter as the cans come "pre hopped" to a level of bitterness.
thanks
Bjorn