I'm interested to know if QGIS 3 can be installed and run side-by-side with a QGIS 2.x installation on windows?
Slightly longer answer
The two applications save all their settings in different places and when they use different versions of the same library, these library versions can safely be installed side-by-side.
It is encouraged and recommended to test QGIS 3.x (either current LTR or current release)
(2.99 while there is no official release) while having a production installations of 2.14 or 2.18 besides to resort to when there are troubles (missing plugins, regressions, ...).
Caveat: project files
The only possible pitfall is, that QGIS projects saved with 3.0 often do not open properly in 2.18. Meanwhile, projects saved with 2.18 can be opened safely with 3.0. To avoid troubles, always work on copies of your 2.x projects when opening them in 3.0. You should do this anyway when you want to open projects with older QGIS versions later on, also for minor releases.
2Followup q: How? Dec 5, 2017 at 21:40
1You should use the osgeo4w installer and install qgis-dev and qgis-dev-pdb– Nathan WDec 6, 2017 at 0:03
5For now, you may miss a lot of plugins when running QGIS 2.99 .– AndreJDec 6, 2017 at 9:30
Sorry, I had to make it a little longer to give some context. Dec 6, 2017 at 10:18
1And how to do it on Linux?– arjanDec 7, 2017 at 15:30