Hi there!
Per the KB at https://kb.netapp.com/Advice_and_Troubleshooting/Data_Protection_and_Security/SnapDiff/FAQ%3A_SnapDiff_Support_in_ONTAP - "SnapDiff v3 is only available to select licensed partners through REST API."
If you're interested in partnering with NetApp, please reach out to me via DM with your corporate contact details and I will have someone get in touch with you.
Hope this helps!