Pretty much. With 3 you can plan and manage things so its workable. With 2, there is really no point even talking about ARG, because it will essentially never happen. A 3rd LHD doesn't give the ADF ARG on day one, but it allow army and navy to put the elements together in future acquisitions. This will never happen with 2, we will always have the wrong mix with 2, because you will never form an ARG.
That would be ideal, but as you mention it would never happen. I too think the F-35B is a distraction, but with 3 you can look at having a small amount of capability. Enough to train, make small deployments, without compromising the core amphibious capability. Realistically I think this would be all the capability Australia would ever need. We really only need enough to cover small operations, until a UK or US carrier is able to turn up.
But you would be getting the 3rd LHD for the amphibious capability. Its for the army.
The ARG concept is very strong, and as you can read the Army is aggressively adopting it, the americans have been suggesting it for decades. However Casually mentioning it would be made up of 2x LHD's and Choules, requiring all three ships to be some how all available, in terms of sustainment they would also need to never break down or require service or maintenance. On top of this, they don't just have to be available for actual operations they also need to be available for training otherwise you will only be training a force element. So all three ships to be available 365 for the next 10 years.
A 3rd LHD doesn't solve all the problems, but will make sure you have your two most important, unique assets available nearly all the time.