Sorry, but it's no longer available for download - why do you need it?
Note: The support policy for OnCommand System Manager 3.1.3 (Offbox) aligns with the support policy for NetApp Data ONTAP operating in 7-Mode. The product will be fully supported until December 31, 2020, and will be available until December 31, 2022. This package will not be available for download from the NetApp Support site after December 31, 2022.
https://kb.netapp.com/Legacy/OnCommand_Suite/How_to_identify_an_OnCommand_product_and_its_capabilities#System_Manager
