Well I came up with this awesome design for a arm for playing triple play, so i built a prototype
That is one sweet arm design. I used to love playing with Knex.
I love the little wheels! Are you riding on 2’s? or are those the 1’s? 
They are 1-3/4" wheels Im hoping to convince everybody on my team that we need to do 9 wheel mini drive next year!
About the design - its a double jointed arm, one joint is at the top of a stationary tower, and the other is joining the 2 arm segments. And the finger bends up to make it easier to pick up tetra’s and hold them more securely - everybody that i have shown this so far say its flicking them off, which wasnt intentional.
The prototype is to scale pretty much, the arms are about the right lengths but they are alot thicker then the actual thing. All the joints rotate freely on the prototype, I didnt dare try to make my k’nex motors run the arms 
PS - weighs 7.6 pounds 