Author: Tobit Flatscher (April 2020)
In order to modify the above code for your robot create a Xacro-file for the parallel robot with a single leg only at first. Your robot is likely under-determined and the end-effector would simply fall to the floor if you would simulate this geometry. Then create the URDF-file from the Xacro macros with this simplified geometry
$ xacro --inorder your_parallel_robot.xacro > your_parallel_robot.urdf
Check the URDF file for its correctness
$ check_urdf your_parallel_robot.urdf
Convert the URDF file to SDF
$ gz sdf -p your_parallel_robot.urdf > your_parallel_robot.sdf
Open the SDF-file and look for the corresponding joint you want to break the closed loop at (I personally think it is generally best to split the parallel robot into serial chains where the end-effector is the last link, keep one of these connections and then introduce the joints between the other legs and the end-effector by SDF-code injection.).
Now introduce the other legs of the parallel robot as serial chains but without the final joints to end-effector (In order to respect URDF's serial chains of course the first chain has to stay connected to the end-effector!). Adapt the syntax that you found in the SDF-file for the joint between the first leg and the end effector to the other final joints by introducing parameters and copy it into your UDRF-file encapsulated in <gazebo> ... </gazebo> tags (last part of model/element.xacro in my code). Furthermore the guides (1 and 2) on the official SDF page might be helpful to understand the SDF-syntax. What is important to know is that the location of the joint in the parent frame is determined by the location of the child and its initial position.