About the Three Treasures of Mahayana (why “Buddha”)

The Great Spirit of the Universe = Buddha
Rationality = law
All sentient beings = monks

It can also be said that Buddha is everything.
It can also be said that it is a function of doing oneself and leading to correct consciousness.
If that's a person, then Masashi. He is a mentor.
However, at the end of the day, Masashi is self-evident and horodomyo, and ❝ the fearless self ❞ is the guiding function.
Therefore, I find my own Buddha nature and Buddha.
If you find a Buddha in your own state rather than an object, it is the law, and it becomes a monk or sanga.
It works without hesitation.

There are various interpretations.

For me, a Buddha is an enlightened person, and it indicates the Buddha and many other Buddhas.
The law is the truth of this world. The Dharma always exists even when there is no Buddha, but it is the Buddha who teaches us the Dharma.
I think monks are people who receive the Buddha's teachings, embody them, and put them into practice.
I think primitive Buddhism was like this.

However, as you said, it is not wrong to view Amitabha and Dainichi Nyorai as laws. Originally it is law, but there is also an interpretation that they dared to embody existence in order to teach us the law.
A body that is the original law is called a body of law, and a body that dares to embody existence is called revenge. Also, the appearance of someone coming down to this world like Buddha is called response.

There's nothing right or wrong. I think it will change depending on how we feel, our environment, and circumstances.
I think the important thing is to live as a monk thinking that the law is always there, and the Buddha can always be by our side.

Note, in primitive Buddhism, it was described above, but after the fall of Buddha, it became sectional Buddhism, and the Buddha changed so that only Buddha was the Buddha, only the Buddha's teachings, and monks were only male monks. This is because Buddha was gradually deified.
Later, those who objected to this appeared. I tried to restore the Three Treasures to their original meaning. There are Buddha statues other than Buddha, the law is the truth, and monks include both women who have become monks and men and women who live in the house. It is said that this was one of the causes of the birth of Mahayana Buddhism.

I read it with interest. Certainly, it may be fair to say that the principal image in that sense is the law, but in my own understanding, I think it should still be made into a Buddha.

What is the word three treasures
There are three ways of thinking: the three treasures in one place, the three treasures of the chief priest, and the present three treasures.
Assuming that Hiro-san said that the original three treasures of the Theravada correspond to the three treasures of the chief priest and the present three treasures
The three treasures of Mahayana Buddhism you mentioned can be taken as either the three treasures of the abbot or the three treasures in one place, but which one is understood?

“In the Three Treasures of Mahayana Buddhism,
Buddha = principal image
Dharma = teachings preached by Buddha or the principal image or ancestor (or Goso or Taisho)
monk = monk (although there's a lot of room for debate here)
I recognize that it is.”
I think this is referring to the Three Treasures of the Chief Priest. After the fall of Buddha, there is a history where Buddha statues were erected as principal images in sculptures, paintings, etc. and used as objects of worship.
On the other hand,
“In terms of nuance, it seems like the principal image is not a 'Buddha' but a 'law'”
Isn't that exactly how to capture the Three Treasures?

I will now state it as my personal opinion.
The fact that the principal image seems to be a law rather than a Buddha is an idea that both Amida Nyorai and Dainichi Nyorai indicate truth, right? But I think it's better to look at “the law” as “teaching to be preached” rather than “truth itself.”
This is because, without teaching, how can we learn that truth? Since truth is beyond words, there is no teaching or anything, and we cannot learn it in the first place. Even if we say that the reality we live in is eternal life itself, we wouldn't understand it if there were no teachings to be taught, right?
That's why Buddha can be an “embodiment of truth,” but then I think it's better to look at the law as “the teaching that has been preached” rather than the truth. It is my own understanding of the Three Treasures in Mahayana Buddhism to see that it is possible to live in the world of Buddha (truth) by learning and touching teachings (law) with friends (monks).

The three treasures have gone one step further and are the space-time itself where they are wrapped into one (?) I think it symbolizes
It's not until you get that far, and depending on the sutras and commentaries, it may be explained that law = truth, so I hope you can further deepen your understanding of Hiro-san from there on.