Sony has been notorious with software updates in the past. In fact, one of the reasons for people not picking up an Xperia device in the past has been their slow burn rate when it comes to rolling out updates. However, off late Sony has started to take the problems of updates seriously and started what is called Concept ROMs. These are basically Beta ROMs that select users will get an update to the latest software releases. Several members who are a part of the Concept ROM community for Sony are already receiving updates to Android 6.0 Marshmallow.
If you are already on the Concept ROM and have not received the update yet, Sony has said that it is rolling out the update gradually so you would be getting it. Not just this, if you have not registered yourself for the Concept ROMs, you can go ahead and do that as there are few more new slots that Sony has opened up. To do this, you will need to install the Sony Concept ROM application from Google Play here.
Once you have registered, and the spots are available, you will be able to download the Marshmallow concept on your Xperia Z3 and Z3 Compact. Sony has stated that only 10,000 spots are available and is open to people living in Sweden, Denmark, Norway, Finland, Iceland, Estonia, Latvia, Lithuania, United Kingdom, Ireland or Germany.
You can read up more on how Sony rolls out concept updates by visiting the official page of Sony for the same here.