Interesting. I have been trying to find info about every bit in Phantasie 3, and have been thinking about making a custom game using the P3 engine. However, much of the stuff seems to be going on the main program file, and it's a bit hard to follow the disassembled code.
I've played through P1 on the Amiga, and as you say the UI is a bit backwards if you're used to P3's. It's a fun game nevertheless when you get into it.
I havent played P2, having never been on the Amiga and all. But I think I will have to give the ST version a go.