Can you keep CC2015.2 and install CC2015.3?
Yes, although the Adobe application manager does not do this as a default.
When choosing to update click on the "advanced options" box as shown in the picture opposite and choose to keep previous versions.
If you do not do this and wish to go back to cc2015.2 use the procedure outlined here: http://www.dvc.uk.com/acatalog/keep-old-premiere.html
As with previous large updates from Adobe, the project format has changed so if you open a project in CC2015.3 it will make a new version of your project which you cannot open in CC20015.2 or earlier.