The question is about all these technologies - though it's about 2 mainly insofar as 2 is an extension of 1.
So the question is why expect any of these technologies to mature on a timescale of decades?
(Or, assuming FOOM, why assume they'd be relatively low-hanging fruit for a FOOMing AI, such that "trick humans into building me nano assemblers" is a prime strategy for a boxed AI to escape?)
As I said, 2 is already here, and it's becoming more here gradually.
For 3, we have a proof of concept to rip of: biological cells. Those also happens to have a specialized assembler in them already; the ribosome. And we can print instructions for it already. There's only 1 problem left and that's the protein folding problem. The protein folding problem is somewhat rapidly made progress on software wise, and even if that were to fail it won't be all that long before we ca simply brute force it with computing power. Now, the other kinds of nanobots are less ...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.