Actutating the Prismatic joints in SimMechanics

hello, Can you help me with the following problem? I have a platform which is connected to the ground using a spherical (S), a prismatic (P) and a rotary (R) joints. There are 3 such legs as shown in the attached image

attached. Only the P joints are actuated. Also I know the position of the P joints at every instant of time. For eg: at time t1, P1=1m, P2=1.2m and P3=1.5m etc. The idea is to move and orient the top platform according to some predefined path by varying the length of the P joins. How can I provide inputs to P joints in Simmechanics to achieve this? I have prepared a CAD model in SolidWorks and using it made a Simmechanics model also.