As for whether 3% is reasonable, again we can look to Epic for evidence. Epic's own Steam competitor takes a 12% cut — and they admitted in court that it was a money-losing venture. That should stop and make you think. The Epic Games Store isn't even a complex ecosystem, it's just a glorified Windows app downloader and even then they couldn't make a profit at 12%.
Apple argues that their 15% fee for most (30% for the ultra-successful) pays for a lot more than just payment services. It pays for absorbing the cost of fraud. It pays for dealing with refunds. It pays for developing the APIs. It pays for employing an enormous team to perform some imperfect-but-useful oversight over the 1,800,000 apps in their store. It pays for a lot of things.
If you think Apple makes too much money, fine. That's a perfectly fine argument to make. That's a very different one to claiming that they're not entitled to make money. Or that the government should dictate prices at them.