Q:

Where is boost-spirit 3? Is it abandoned?

I can't find any download for boost spirit 3.
Seems like the official website stopped talking about it at the end of December? Where did it go?

A:

The source code is available on Github. Documentation is here. Quoting from the Blog:

C++ Now 2015. Today is the official release of Spirit X3, aka Spirit
  3.0.0. X3 will be in beta, coexisting side by side with Qi, Karma, Lex and Classic.

According to Sehe (see comments), X3 was included in the Boost release at least as far back as November 2015. Since it's header only though, I figure you could just as easily download Spirit from Github.
If you look in the 'Include' section of the documentation (linked above), you'll read:

Spirit is a header file only library. There are no libraries to link
  to. To use Spirit X3 simply include:
#include <boost/spirit/home/x3.hpp> To distinguish between Spirit versions, you can inspect the version file:

#include <boost/spirit/home/x3/version.hpp> using the preprocessor define

SPIRIT_X3_VERSION It is a hex number where the first two digits
  determine the major version while the last two digits determine the
  minor version. For example:
#define SPIRIT_VERSION 0x3000 // version 3.0

Peek in Github and you'll find include/boost/spirit/home/x3. If you check the history of pull requests you'll see the project is very much alive. If you have doubts about the activity of the project, contact the team.

