According to retired famous leaker @evleaks, Moto Maker will support Moto G 3rd Gen. Moto Maker Program was launched in 2013 by Motorola to allow customer to customized the design of their Moto X smartphone.
Using Moto Maker, customer may able to choose different color of back panel, front panel, trim, cases even capacity of internal storage. Customer also can choose or select the wallpaper, greetings and type chargers for their smartphone.
Considering Moto G is not at the top rank, some features may not be available but however, if this leaks is true, this will be a huge jumps over Moto G 2014.
Take a look of the image below to see how this customization may work: