Why 3 g of tea, 150 cc of water, 6 minutes?

I would not assume this is really an International standard. From what I have learned of Asian or Gongfu brewing this is nothing like what is recommended. It would approximate the standard for Western brewing though six minutes is a long steep.

From my research, it is indeed an international standard for evaluating tea – ISO 1303. Not necessarily an international standard for making the best drinkable cup of any particular type of tea, which would be hard to standardize. :)

This sounds like the practice known as “cupping”. I.e. a method to evaluate teas on an even ground by using identical teaware and overbrewing parameters much like the ones described. Marshaln has a good article about this: http://www.marshaln.com/2017/03/whats-a-good-tea/

From what I know about professional tea tasting, at least in Japan, the idea is to standardize and also to be able to tell flaws in the process, as well as quality. But it won’t taste nor smell how the consumer would drink it. For example, all green tea is brewed in boiling water.
It other words, it’s mostly for professional tea buyers, and a lot of practice is required. I don’t think that the average tea drinker really needs to taste tea like that.