I’ve never really used the statistics sensor so just now learning it but…
I think you will need to define the number of samples in order to use the “state_characteristic: change” option.
the description for that option says:
The difference between the oldest and newest measurement stored.
Which tells me that it will use the value of the first sample and then the value of the last sample. then since the default “sample_size” is 20 (and you haven’t specified a different value) then it will look at value of sample 1 and then the value of sample 20 and subtract the two to get the “change” value.
Since your sensor updates every 20 seconds it will take 20 seconds * 20 samples = 400 seconds = 6 min 40 seconds to get that pour value change.
since it updates every 20 seconds you could change the sample size to 2 and then assume it takes less than 20 seconds to pour a glass of beer. That could get you the pour value change.
However, I’m not sure what would happen if the sensor updated right in the middle of your pour. That would cause the value to be off and you wouldn’t get the correct value in the notification. Or you might get 2 notifications - one that says “you just poured 4 oz” followed some seconds later by another notification that says “you just poured 6 oz” (for a single 10 oz pour divided over 2 pour samples). I don’t know how to solve that problem using a statistics sensor. Maybe there is but I just don’t know enough about it.
…thinking on this for a while now…I’m not sure you will be able to do what you want.
there are too many variables to account for.
You already said that you needed to use a change greater than 3 oz due to variations in the sensor. (BTW, you have the numeric_state trigger ion your automation backwards. It should be “above: 3” not “below: 3”).
how long in time is a pour? a small one is a shorter time than a bigger one.
how long is there between pours? if it’s just you then that time will likely be a while (hopefully… ). If there are 4 of you pouring drinks it could be pretty often.
None of the solutions I can think of will ever be able to take all of those variables into account and give you a real-time notification of how much the last pour amount was to any degree of accuracy - especially if your sensor is ±3 oz at any point in time.