Powering Arduino + raspberry pi 3 using one voltage source(Battery)
Connect your batteries in series and use a DC to DC Buck Converter to regulate the voltage to 5V.  You can then power both devices from the Buck Converter output or just simply let the Pi Power the Arduino using the USB to USB connection.  The DC to DC Buck converters can be purchased cheaply on Ebay or Amazon like this one:  DCtoDC Buck Converter