There's a vast difference between being an atheist and being part of a non-deistic faith. Bhuddism is a religion, it has dogmas, it has a supernatural understanding of the world, it has concepts like Hell, saints, and more, one cannot be a Bhuddist while at the same time being an atheist. It's an oxymoron.
Atheist and non-deistic are different words for the same experience.
Buddhism is not a monolithic belief system either, which is why I chose a school like Therevada that is atheistic. It isn't the only one, Zen is probably the most famous in this aspect, but to come onto your next bit.
In the schools of Buddhism that do have what westerners commonly call "Deities", what Buddhists call deities or saints are not a 1:1 like for like to Christian/Islamic mythology. If anything, they're closer to Norse mythology. Gods are very long lived, but they are mortals. They die.
A Buddhist deity is trapped in Samsara just like every other living thing. Like Mormons, a good human can become a God. But (unlike Mormons) a bad God could become an ant next time. This, quite common concept of polytheism in Buddhist countries like Tibet, is hardly equivalent to a Muslim belief in an eternal singular deity.
Similarly, the Bodhisattva (Saints) are not subservient to deities. They're super-humans and Buddhism generally promotes the idea that humanity is superior to divinity because Gods find it harder or cannot obtain enlightenment. Again, another belief that doesn't align well with Western theism and "Ethics and faith go hand in hand". The Buddhist saints are not God, creators or anything like "The one God" in other religions.
It's quite easy to be a Buddhist and reject belief in "a God". The Buddhists who do believe in deities also generally don't believe they're infallible, omniscient or all good; and don't tend to attach a particular set of dogmas to them either.
That's because the consumerism characteristic of post-WW2 Japan (and the authoritarianism of the State Shinto that preceeded it) basically hollowed out any actual belief in Shinto. It's functionally as dead as roman paganism but just with orders of magnitude more people who claim to follow itit.
Be that as it may, it is still hard to suggest this isn't the main traditional religion of the Japanese people.
As someone not of European heritage, I found the idea that a God can be three but yet one very strange. I similar found the idea that a God can be the creator of all, even evil, and still be all good an oxymoron but that clearly isn't a barrier for most Christians/Post Christians.
Atheism isn't typical of post-Roman Middle-Eastern religion true, but theism is not typical of non-western religions. Even where Westerners have slapped the term "God" on terms like Kami, Shén etc: these entities do not possess all, or sometimes any, of the traits a theist would expect a God to have.
These "Gods" are neither all good or all bad. They eat, they fuck, they can make mistakes and they die. All concepts foreign to the idea of a Western "God".
