The third version, of which Boston Dynamics released a video on YouTube on Feb. 12, looks exactly like the second except with a large arm-like appendage attached to the top. As the title of the video (below) aptly suggests, this arm can be used to lend a hand to its armless friend by opening doors for it.
The robot does appear to struggle with the weight of the door for a moment, having to really brace against the door before pushing it open. However, everything else goes down smoothly, and the robot is able to hold the door for its companion. Additionally, the arm rotates at its base, allowing the robot to keep holding the door open for itself as it follows its buddy through.