If you want to learn assembly language, keep in mind that assembly is processor-specific. Since there are not TR-3200 tutorials, if you want to learn about assembly, you will have to learn from other processors. While you won't learn TR-3200 specifics, I think that anyone could benefit from learning any type of assembly, since it will give you a very good overview of the aspects of assembly that are the same across the board.
That out of the way, I'd like to introduce you to the subject that first got me into assembly: The Nintendo Entertainment System. Game for the NES were written in assembly for the 6502 processor. What cooler way of learning assembly is there than writing games? Here's the first NES tutorial that I used: http://www.nintendoage.com/forum/messageview.cfm?catid=22&threadid=7155
I believe that it's a great start to assembly. Hell, I've since made several NES games in assembly.
While the 6502 processor is not nearly as powerful as the TR-3200, its strength for beginners lies in its simplicity. A small set of opcodes (http://www.obelisk.demon.co.uk/6502/reference.html
), it was a RISC before RISC was a thing.
I say: Learn the basics of assembly by doing something fun, and then the TR-3200 will make more sense.