Have Jython and IronPython reached compatibility with Python 3 yet? Last I checked they hadn't, or it was still in alpha.

Yes, but ain't nobody really using Jython or IronPython. Yes some people are, but I'd bet a very small group of the greater Python whole. You also introduce incompatibilities that way. Perl6 has this in the base implementation, so I don't have to switch to a lesser maintained distribution.

I don't know, but Python 3 still isn't relevant to many people.

For example, I have to explicitly install 2.7 to be able to use Cocos2d-x build scripts.

Maybe, but it seems like a bad idea to invest in Python 2 (which is teaching is ended of life) just to get parallelism when there are other programming languages that have great concurrency stories and a bright future. That said, if PyPy3 ditches the GIL, let me know!

Someone posted an article awhile back...people are moving over by a significant amount now.

Yeah, "not relevant to many people" is a wildly false claim. If it were anywhere near true, we wouldn't see the level of Python 3 support that we have in the major packages (95.8%).

http://py3readiness.org/

Jython project's latest release and news post was mid-2015; the project would seem to be pining for the fjords, so to speak.

Jython and IronPython are both limited to Python 2.x, ZipPy is WIP without a stable release.