Sony does a pretty good job of sticking closely to stock Android. They are also pretty big on providing developer friendly devices. If you want a fully stock experience though you will need to resort to running a Rom. Sony has been working on concept roms for their devices for a while now. They have a Concept version of Android 6.0.1 AOSP Marshmallow for the Xperia Z3 Compact.
Installing the concept rom is not the simplest thing to do so if you don't have any experience with Roms at all you may want to skip this one until you get some practice. The install requires a whole host of processes including backing up the TA partition, installing some files with the flashtool, and then there are several other steps. For the full in depth guide head to the link below.
via Droid Views