First you dont stack cards of similar name you just play them somewhere in your support area.
The main answer is best described by saying look at each card seperately. If a hobbit loses a skirmish look at
Sudden Fury 1. It says place a token here. So you do. Then you look at the
Sudden Fury 2. It says place a token here so you do. Repeat 4 times.
So yes 3 hobbits lose and you get 12 tokens.
When you remove a token you get 1 strength from each token. This is because you cannot combine actions that start a bolded phase word like
Skirmish: or
Response:. Using a line of text like this is unique to that one action. You remove 1 token from
Sudden Fury 3 and thats just relevant to the gametext of that
sudden fury. You have not removed a token using the gametext of
Sudden Fury 1 so it does not count.
So those 12 tokens are wroth 12 strength in summary
Cheers