what's the true difference between 0.12m and ph3?
0.12m is no longer actively supported by the developer. As such, it is relatively outdated compared to ph3. (For instance, ph3 supports actual Full Games, Pixel Shaders)
On the other side, 0.12m has more tutorials than ph3 does... but the difference between 0.12m and ph3 code wise isn't that far off. There's still @Mainloop, @Initialize, etc.
The key difference is that 0.12m does a lot of hand holding, but is also really restrictive because of it. With ph3, it is, by far, more easily modified compared to 0.12m. And personally, I think it's fairly easy to grasp what's happening with the default samples provided, and if there's any questions there's always the pinned Q&A thread.
Also, with ph3, you're likely to end up using tasks a lot more. You'd also preferably want to learn how to use tasks instead of using @Mainloop, but if you're just starting out, you don't have to really worry about it until you get the hang of it.
Also, if you're barely starting with danmakufu, I strongly suggest starting with ph3, as it is still being updated. There are currently efforts to improve the amount of tutorials for it, as well.