Task 40770: 1. Update version variables for v7.0.0-preview3 release (dotnet#3825)

The outstansing PR pipeline checks are transient problems, nothing to do with changing a '2' to a '3', so merging to avoid release delays.

Author: paulmedynski

eng/pipelines/libraries/common-variables.yml

@@ -36,7 +36,7 @@ variables:
   - name: Preview
     value: '-preview'
   - name: Revision
-    value: '2'
+    value: '3'
 
   - name: NugetPackageVersion
     value: $(Major).$(Minor).$(Patch)