Hello.

When you need to define kinematics for equipment with an extendable arm, such as lift cranes, using Kinematic Relations is one of the solutions. 

Here is the sample for using Kinematic Relations. J1, J2, J3 can extend each 1,000mm in this model. Finally J3 can extend to 3,000mm.

For this, you can use the function, Ramp() in Kinematic Relations.

When you search this in User Assistance, you can find this explanation.

You can apply this to the sample model.

After defining prismatic joints from the Fix part to each Joint, define Kinematic Relations.

Please, refer to the video for the detail procedure and the 3dxml file made on cloud.

Thank you.