How to install PGF/TikZ 3.0 in MikTeX?

As of now, (probably actually a few months back.)

PGF 3 is in MikTeX.

Open up the MikTex Package Manager and Click remove on the pgf package, (if you have it already) then click add again to get the new version, or run the update wzard

Download the tds.zip https://sourceforge.net/projects/pgf/files/latest/download?source=files

Unpack the zip file somewhere outside your miktex system. Then add the root folder as a new root in miktex settings. When pgf is available throught the package manager you can simple remove the root in miktex settings again.