The bluetooth on the RPI3 need to comunicate with the main processor by UART so they choose to use a “mini UART” on the official uart pinout. The “mini UART” don’t have fixed speed communication and this is not working with robotis motors. To use Pixl you will need to “overlay” the uart of the bluetooth, to be more explicit you need to disable your bluetooth to communicate with dynamixel motors through a Pixl.
Raspberry do something to simplify the workaround from the Raspbian Jessie release dated of 2016-03-18.
If you have this realease or a upper one use this tutorial, else use this one.
We work on a new board called Hipi who permit to use bluetooth and communicate with many kind of robotis servos.