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Hello,
Based on my research, you can use Windows 2012 as upstream WSUS 3.0 as downstream. But you should avoid use WSUS 3.0 as upstream and Windows 2012 as downstream.
Here is a similar discussion for your reference:
https://social.technet.microsoft.com/Forums/windowsserver/en-US/9e79bf50-48e6-401a-9f2a-1783e5438efb/upstreamdownstream-server-wsus-versions?forum=winserverwsus
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