After updating many of its smartphones to Android Marshmallow, Motorola has started to push the latest version of the OS to Moto G 3rd Gen users. The update seems to be available for only the users from the USA and Canada. Additionally, it also appears like it is currently available only on the unlocked (XT1540) variant of the Moto G 3rd Gen.
The update is said to be around 2GB in size and bring all the well-known features of Android Marshmallow. Some of them are the Doze mode, Google Now on Tap, granular app permission among others. Since it is a Motorola device, you will also see a few features which are a part of almost every update rolled out by the company.
As I already mentioned that update seems to be available only for the unlocked variant, but you can expect it to be available in other regions and for carrier variants in near future as well.