Open source CPUs are a pipe dream because of the massive cost required to develop and fab silicon. It might be workable if you went back some generations and utilised some old ex-patent architecture. iirc China has done something like this, basing homegrown processors based on MIPS. You cant do DIY hardware in your garage/basement/shed/whatever like you can with software.
Not yet. What are the chances of 3D printing making this sort of thing a reality?
Zero, at least for the foreseeable. You'd be talking about 3D printing at a microscopic to atomic level. Thats several generations (read decades) of 3D print technology away, there probably would be home photolithography before that. Bear in mind today's semiconductor fabrication plants cost several billion to build, its not about to scale down for homebrew.
I'll take your word for it as its not my area of expertise, but sometimes technology moves quicker than expected (if there is a breakthrough in one particular field which then ripples through all of the others)