That extra block is strange. What is the purpose of it? Some how, they arms need to go onto that block. And here is the thing to pay attention to: The spacing between the arms needs to be identical to the spacing between the other two sets of arms. Also, the distance from the center of the effector needs to be identical with the other two sets of arms.

There is only going to be one location where the arms can go. But that extra block does not make sense.